Methods for the synthesis of dicarba bridges in peptides

ABSTRACT

A method for preparing a peptide or peptides containing a dicarba bridge, comprising: (i) providing a reactable peptide having at least two complementary metathesisable groups or two or more reactable peptides having at least two complementary metathesisable groups between them; (ii) subjecting the reactable peptide or reactable peptides to metathesis to form a reactable peptide or peptides having at least one unsaturated dicarba bridge; and (iii) adding one or more further amino acids to one or both ends of at least one of the reactable peptides.

FIELD OF THE INVENTION

The present application broadly relates to dicarba peptide analogues, and methods for the synthesis of these analogues via alternating solid phase peptide synthesis and catalysis.

BACKGROUND TO THE INVENTION

Peptides and proteins are oligomers of amino acids that mediate a diverse array of functions in all living systems. Virtually all processes that sustain life in organisms are carried out by proteins. Peptides and proteins exist as hormones, biochemical inhibitors, antigens, growth factors and transmembrane carriers. The high biological activity and specificity of many peptides makes them particularly attractive pharmaceutical targets and improved synthesis techniques have allowed peptide therapeutics to become a commercially viable option in recent years. In combination with advances in chemical modification, administration and formulation, the pharmaceutical market has seen a dramatic increase in the number of peptide-based therapeutics introduced each year. In the early 1990s, peptides accounted for approximately 5% of the new chemical entities introduced onto the drug market. Today this figure has grown to approximately 25% and there are currently over 40 peptide-based drugs on the market and approximately 300 in clinical trials. Peptide therapeutics are produced via recombinant and synthetic means, with some preparations utilising a combination of both approaches. It is expected that improvements in DNA technology will one day allow large scale recombinant synthesis of peptide sequences containing non-coded amino acids.

Despite their beneficial properties, the development of orally active peptide drugs for clinical application has been severely restricted by their unfavourable physicochemical properties. Their poor resistance to proteolytic enzymes, rapid excretion from the body through, for example, the liver and kidneys, their inability to cross membrane barriers, such as the intestinal and blood-brain barriers, and in some cases, their low solubility and tendency to aggregate, have contributed to the poor bioavailability of peptide-based therapeutics. The successful therapeutic application of many peptides, and in particular their oral delivery, requires the design and synthesis of novel peptidomimetics which possess improved physicochemical properties and uncompromised biological activity.

Various peptidomimetic chemistries have been developed in recent years and these now provide a toolbox for researchers to investigate ways of enhancing the therapeutic potential of unmodified native sequences. Researchers are looking to design peptide mimics that retain all the beneficial pharmacological properties of their natural counterparts, whilst addressing the negative aspects relating to their physicochemical stability. Several classes of pseudo-peptides have been developed and investigated with respect to their chemical properties, physiological activities and ease of chemical synthesis. Many concentrate on enhancing metabolic stability and bioavailability, whilst others focus on restricting conformational flexibility in order to increase receptor selectivity. These include β- and γ-peptides, peptide nucleic acids, peptoids, stapled peptides, cyclic analogues, retro-inverso isomers of naturally occurring sequences, and peptides that incorporate non-proteinaceous amino acids. Each class of pseudo-peptide claims a unique set of properties, such as high specific binding and enhanced activity at the target receptor, improved transport properties through cellular membranes and more often than not, reduced rates of degradation by peptide-specific enzymes.

Inspite of recent progress, the successful therapeutic application of peptides still requires the design and synthesis of novel peptidomimetics which possess improved physicochemical properties and uncompromised biological activity.

Typically, a peptidomimetic seeks to advantageously modify the molecular properties of a native peptide, such as its stability or biological activity, through modification of the native peptide or by using similar systems or scaffolds to mimic the peptide whilst simultaneously closely replicating the secondary and tertiary structure of the native peptide. A metabolically unstable motif such as a disulfide bond (—S—S—) may be replaced with a stable mimetic having enhanced activity and/or enhanced stability for in vivo administration. The peptidomimetic will only be useful, however, if activity is maintained and key receptor interactions are preserved. In this respect, it is important for peptidomimetics to replicate the secondary and tertiary structure of the native peptide to a large degree.

The use of dicarba (—C—C—) containing bridges in peptidomimetics, in particular as substitutes for disulfide bridges, appears to have great potential. Dicarba bridges are not as reactive as disulfide bridges. Replacing a disufide bridge with a dicarba bridge has the potential to produce compounds having the activity of, or similar activity to, the disulfide-containing polypeptides, but with better biostability. Dicarba peptidomimetics can be prepared via homogeneous catalysis. Although modern catalysts are capable of highly chemo-, regio- and stereoselective reactions, for some peptide sequences, ring closing metathesis (RCM) can be problematic resulting in low yields or no dicarba peptide formation.

Alkene metathesis provides a versatile method for the formation of C═C bonds via a metal-catalysed molecular exchange of alkylidene fragments between two olefins. Using alkene metathesis, it has been possible to replace native disulfide bridges with stable dicarba bridges but the resulting stereochemistry across the newly introduced C═C bond of the dicarba bridge can be difficult to control. The dihedral and torsional angles of a cystine bridge differ from those of a dicarba bridge and it is important to conserve the structural conformation of the peptide in order to replicate native activity. Furthermore, the formation of a dicarba bridge in a peptide or between two peptides can be difficult, particularly in longer and more complex peptides. Consequently, the synthesis of larger amounts of longer and more complex dicarba-bridge containing peptides is often not feasible. This is a particular hinderance to research as it deters investigators from researching the potential therapeutic applications of such peptides.

There is a clear need to develop improved methods for preparing peptide analogues and peptide mimetics to accommodate a growing list of recalcitrant peptide sequences. There is a need for a method of preparing peptide compounds in which there is some degree of control over the geometry of the newly installed dicarba bridge in order to conserve the structural features of the native peptide while simultaneously maintaining the activity of the native peptide. Furthermore, there is a need for improved methods for producing longer and more complex peptides comprising dicarbe bridges.

SUMMARY OF THE INVENTION

This application relates to a method for preparing a peptide or peptides containing a dicarba bridge.

According to one embodiment, there is provided a method for preparing a peptide or peptides containing a dicarba bridge, comprising:

-   (i) providing a reactable peptide having at least two complementary     metathesisable groups or two or more reactable peptides having at     least two complementary metathesisable groups between them; -   (ii) subjecting the reactable peptide or reactable peptides to     metathesis to form a reactable peptide or peptides having at least     one dicarba bridge; and -   (iii) adding one or more further amino acids to one or both ends of     at least one of the reactable peptides.

According to another embodiment, there is provided a method for preparing a peptide containing a plurality of dicarba bridges, comprising:

-   (i) providing a reactable peptide having at least two complementary     metathesisable groups; -   (ii) subjecting the reactable peptide to metathesis to form a     reactable peptide having at least one dicarba bridge; -   (iii) adding one or more further amino acids comprising at least two     complementary metathesisable groups to one or both ends of the at     least one peptide; and -   (iv) repeating steps (ii) and (iii) at least once.

A dicarba bridge is formed by subjecting two complementary metathesisable groups to metathesis. The two complementary metathesisable groups may be provided in a single reactable peptide so that subjecting the reactable peptide to metathesis results in the formation of an intramolecular dicarba bridge. Alternatively, the two complementary metathesisable groups may be provided between two or more reactable peptides, which are each independently the same or different. Metathesis of the two reactable peptides results in the formation of an intermolecular dicarba bridge.

In a preferred embodiment, the step of adding one or more further amino acids to one or both ends of at least one of the reactable peptides is performed by peptide synthesis. In this approach, the one or more further amino acids are added to the peptide sequentially. Preferably, the one or more further amino acids are added to the N-terminus of the reactable peptide or peptides.

In a preferred embodiment, one of more of the reactable peptides may be provided on a solid support. Where the reactable peptides are provided on a solid support, the step of adding one or more further amino acids to one or both ends of at least one of the reactable peptides is performed by solid phase peptide synthesis (SPPS).

The present invention provides a method which can enhance the performance of dicarba bridge formation in complex peptides and/or enhance the performance of ring closing metathesis between two complementary metathesisable groups via alternating steps of peptide synthesis and catalysis.

Additionally, the alternating steps of peptide synthesis and catalysis of the present method can be used to introduce metathesisable groups into a sequence in a stepwise fashion. Such a technique may remove or eliminate the need for orthogonal protecting group strategies. The technique also assists the catalysis of ‘difficult’ sequences (e.g. residues which promote deleterious aggregation, unfavourable conformations, and poor peptide solubility and reagent penetration) by allowing troublesome sections of the sequence to be omitted until after the scheduled catalysis has been performed. The omitted residues are added to the truncated sequence via peptide synthesis following catalysis.

Catalysis refers to metathesis to form an unsaturated dicarba bridge, and optionally reduction of the newly formed dicarba bridge. Accordingly, the method may also comprise the step of reducing at least one unsaturated dicarba bridge to form an alkene-containing dicarba bridge or a saturated dicarba bridge. Preferably, the reduction is steroeselective and the alkene-containing dicarba bridge that is formed by reduction is enriched in the cis- or trans-isomer.

This reduction step may occur either before or after one or more further amino acids are added to at least one peptide. Preferably, the reduction step may involve either hydrogenation of at least one dicarba bridge or hydrosilylation and protodesilylation of at least one dicarba bridge. Formation of dicarba bridges involves the use of complementary pairs of metathesisable groups. A metathesisable group is a functional group that can undergo metathesis when unblocked or in an activated state.

The formation of a dicarba bridge in a linear peptide may, in some instances, be difficult. Therefore, a reactible peptide may include a turn inducing residue located between the at least two complementary metathesisable groups. This turn inducing residue may further enhance the performance of ring closing metathesis between two complementary metathesisable groups.

In another embodiment of the method, the reactable peptide may comprise a removable tether between at least two complementary metathesisable groups, which is removed after at least one unsaturated dicarba bridge is formed to provide a peptide containing an intermolecular dicarba bridge. The use of the removeable tether may further enhance the performance of cross metathesis between two complementary metathesisable groups on two reactable peptides.

Blocking groups may also be used to install multiple dicarba bridges in a peptide or peptides. In a preferred embodiment, the reactable peptide has three or more complementary metathesisable groups, or at least two reactable peptides have three or more complementary metathesisable groups between them, where two of the complementary metathesisable groups are unblocked and form the dicarba bridge in step (ii), while the other metathesisable group or groups are blocked. The method may further comprise the step of unblocking at least one blocked metathesisable group. A further amino acid or further amino acids may then be added to one or both ends of the reactable peptide or peptides, and the unblocked metathesisable groups subjected to a further metathesis step to form a further dicarba bridge. The method may still further comprise the step of adding at least one further reactable peptide having at least one unblocked metathesisable group, and subjecting the reactable peptides to metathesis to form a further dicarba bridge.

The present invention also provides a peptide or peptides with at least one dicarba bridge when synthesised by the method described herein.

This approach may also be combined with the other preferred features described herein. These approaches may be used separately or in any combination with the method of the present invention.

BRIEF DESCRIPTION OF THE FIGURES

The invention is described further by way of example with reference to the accompanying drawings in which:

FIG. 1 is a schematic diagram showing a method for preparing a peptide containing a dicarba bridge in which a peptide containing two metathesisable groups is synthesised on a solid phase, and the method involves subjecting the reactable peptide to alternating steps of catalysis and solid phase peptide synthesis (SPPS) before the peptide is cleaved from the solid phase, according to an embodiment of the invention.

FIG. 2 is a schematic diagram showing a method for preparing a peptide containing a plurality of dicarba bridges in which a peptide containing two metathesisable groups is synthesised on a solid phase, and the method involves subjecting the reactable peptide to alternating steps of catalysis and solid phase peptide synthesis (SPPS) to form a plurality of dicarba bridges. When x dicarba bridges are formed, the peptide is cleaved from the solid phase, according to an embodiment of the invention.

FIG. 3 is a schematic diagram showing a method for connecting several peptides together in which one peptide is synthesised on a solid support and the method comprises alternating steps of catalysis and solid phase peptide synthesis (SPPS) to extend the first peptide, according to an embodiment of the invention.

FIG. 4 is a schematic diagram showing a method for forming an intermolecular and an intramolecular dicarba bridge in which two reactable peptides, each containing one metathesisable group are provided, one of which is synthesised on a solid support, and the method involves subjecting the reactable peptides to catalysis to form an intermolecular dicarba bridge and alternating steps of solid phase peptide synthesis (SPPS) and catalysis to form an intramolecular dicarba bridge before the peptide is cleaved from the solid phase, according to an embodiment of the invention.

FIG. 5 is a schematic diagram showing a method for forming an intermolecular dicarba bridge in which a first peptide containing at least one metathesisable group is synthesised on a solid support, and the method involves subjecting the first peptide and a second peptide containing at least one metathesisable group, to alternating steps of catalysis to form an intermolecular dicarba bridgeand solid phase peptide synthesis (SPPS) to extend the second peptide before the peptide is cleaved from the solid phase, according to an embodiment of the invention.

DETAILED DESCRIPTION

As described above, this application relates to a method for preparing a peptide or peptides containing an unsaturated dicarba bridge. The method comprises synthesising a reactable peptide having at least two complementary metathesisable groups or one or more reactable peptides having at least two complementary metathesisable groups between them. The reactable peptide or reactable peptides are then subjected to metathesis to form a reactable peptide or peptides having at least one unsaturated dicarba bridge. Once the bridge has been formed, it can optionally be treated to further catalysis steps (such as reduction of the unsaturated dicarba bridge) before the one or more further amino acids are added to either end of the at least one reactable peptide. Reduction of the formed dicarba bridge may form a saturated dicarba bridge or an alkene-containing dicarba bridge. The alkene-containing dicarba bridge may be enriched in the cis- or trans-isomer.

Types of Compounds and Groups

The term “amino acid” is used in its broadest sense and refers to L- and D-amino acids including the 20 common amino acids such as alanine, arginine, asparagine, aspartic acid, cysteine, glutamic acid, glutamine, glycine, histidine, isoleucine, leucine, lysine, methionine, phenylalanine, proline, serine, threonine, tryptophan, tyrosine and valine; and the less common amino acid derivatives such as homo-amino acids (e.g. β-amino acids), N-alkyl amino acids, dehydroamino acids, aromatic amino acids and α,α-disubstituted amino acids, for example, cystine, 5-hydroxylysine, 4-hydroxyproline, α-aminoadipic acid, α-amino-n-butyric acid, 3,4-dihydroxyphenylalanine, homoserine, α-methylserine, ornithine, pipecolic acid, ortho, meta or para-aminobenzoic acid, citrulline, canavanine, norleucine, δ-glutamic acid, aminobutyric acid, L-fluorenylalanine, L-3-benzothienylalanine and thyroxine; β-amino acids (as compared with the typical α-amino acids) and any amino acid having a molecular weight less than about 500. The term amino acids can also include non-natural amino acids such as those described in U.S. Pat. No. 6,559,126, which are incorporated herein by reference. The term also encompasses amino acids in which the side chain of the amino acid comprises a metathesisable group, as described herein. Further, the amino acid may be a pseudoproline residue (ψPro).

The term “side chain” is used in the usual sense to refer to the side chain on the amino acid, and the backbone to the H₂N—(C)_(x)—CO₂H (where x=1, 2 or 3) component, in which the carbon in bold text bears the side chain (the side chain being possibly linked to the amino nitrogen, as in the case of proline, and other structural analogues).

The amino acids may be optionally protected. The term “optionally protected” is used herein in its broadest sense and refers to an introduced functionality which renders a particular functional group, such as a hydroxyl, amino, carbonyl or carboxyl group, unreactive under selected conditions and which may later be optionally removed to unmask the functional group. A protected amino acid is one in which the reactive substituents of the amino acid, the amino group, carboxyl group or side chain of the amino acid are protected. Suitable protecting groups are known in the art and include those disclosed in Greene, T. W., “Protective Groups in Organic Synthesis” John Wiley & Sons, New York 1999 (the contents of which are incorporated herein by reference) as are methods for their installation and removal.

The amino group of the amino acid may be optionally protected. Preferably the N-protecting group is a suitably protected salt or carbamate such as, 9-fluorenylmethyl carbamate (Fmoc), 2,2,2-trichloroethyl carbamate (Troc), t-butyl carbamate (Boc), allyl carbamate (Alloc), 2-trimethylsilylethyl (Teoc) and benzyl carbamate (Cbz), more preferably Fmoc.

The carboxyl group of the amino acid may be optionally protected. The carboxyl protecting group is preferably an ester such as an alkyl ester, for example, methyl ester, ethyl ester, t-Bu ester or a benzyl ester.

The sidechain of the amino acid may be optionally protected. For example, the carboxyl groups of aspartic acid, glutamic acid and α-aminoadipic acid may be esterified (for example as a C₁-C₆ alkyl ester), the amino groups of lysine, ornithine and 5-hydroxylysine, may be converted to carbamates (for example as a C(═O)OC₁-C₆ alkyl or C(═O)OCH₂Ar aromatic carbamates) or imides (such as phthalimide or succinimide), the hydroxyl groups of 5-hydroxylysine, 4-hydroxyproline, serine, threonine, tyrosine, 3,4-dihydroxyphenylalanine, homoserine, α-methylserine and thyroxine may be converted to ethers (for example a C₁-C₆ alkyl or a (C₁-C₆ alkyl)arylether) or esters (for example a (C═O)C₁-C₆ alkyl ester) and the thiol group of cysteine may be converted to thioethers (for example a C₁-C₆ alkyl thioether) or thioesters (for example a C(═O)C₁-C₆ alkyl thioester).

By a “peptide” is meant any sequence of two or more amino acids, regardless of length, post-translation modification, or function. “Polypeptide”, “peptide” and “protein” are used interchangeably herein. The peptides or mimetics thereof of the invention are typically, though not universally, between 4 and 90 amino acids in length. In various embodiments a peptide of the invention may be less than 200 amino acids in length, less than 180 amino acids in length, less than 160 amino acids in length, less than 140 amino acids in length, less than 120 amino acids in length, less than 100 amino acids in length, less than 90 amino acids in length, less than 80 amino acids in length, less than 70 amino acids in length, less than 60 amino acids in length, less than 50 amino acids in length, less than 40 amino acids in length, less than 35 amino acids in length, less than 30 amino acids in length, less than 28 amino acids in length, less than 27 amino acids in length, 25 amino acids in length, less than 20 amino acids in length, less than 18 amino acids in length, less than 15 amino acids in length, less than 10 amino acids in length, or about 4 or 5 amino acids in length.

“Polypeptide” as used herein refers to an oligopeptide, peptide, or protein. Where “polypeptide” is recited herein to refer to an amino acid sequence of a naturally-occurring protein molecule, “polypeptide” and like terms are not meant to limit the amino acid sequence to the complete, native amino acid sequence associated with the recited protein molecule, but instead is meant to also encompass biologically active variants or fragments, including polypeptides having substantial sequence similarity or sequence identity relative to the amino acid sequences provided herein.

One class of peptides of interest are the peptidomimetics—that is, a peptide that has a series of amino acids that mimics identically or closely a naturally occurring peptide, but with at least one dicarba bridge. The dicarba bridge may, for example, replace one or more naturally occurring disulfide bridges or replace one or more non-covalent interactions present in the peptide, (such as salt bridges, hydrogen bonds, ionic bonds, van der Waals forces or hydrophobic interactions) which may be involved in secondary structure motifs such as α-helices or β-sheets, or the formation of 3-dimensional structural features of the naturally occurring peptide. In identically or closely mimicking a naturally occurring peptide, the peptidomimetics having at least one dicarba bridge may optionally include one or more differences from the natural peptide, such as the removal of a cystine bridge, a change by up to 20% of the amino acids in the sequence, the inclusion of non-natural amino acids, D-amino acids or β-amino acids as non-limiting examples. Of particular interest are dicarba analogues of naturally-occurring disulfide-containing peptides, in which one or more of the disulfide bonds are replaced with dicarba bridges. These may also be classed as pseudo-peptides.

A “dicarba analogue” refers to a peptide which contains an amino acid sequence corresponding to a naturally occurring or synthetic peptide, but containing a dicarba bridge—either as an addition to the peptide, or as a substitution for one or more of the bridged cystine-amino acid residue pairs. “Dicarba-substituted” analogues, which are analogues of naturally-occurring or synthetic peptides, but with one or more of the disulfide bridge forming cystine amino acid residue pairs, are a subclass of particular interest. A notable subclass of the dicarba analogues are the mono-dicarba analogues, and the bis- and higher dicarba analogues.

The amino acid or peptide may be in the form of a free compound, or in the form of a salt, solvate, derivative, isomer or tautomer thereof.

The salts of the amino acids or peptides are preferably pharmaceutically acceptable, but it will be appreciated that non-pharmaceutically acceptable salts also fall within the scope of the present invention, since these are useful as intermediates in the preparation of pharmaceutically acceptable salts. Examples of pharmaceutically acceptable salts include salts of pharmaceutically acceptable cations such as sodium, potassium, lithium, calcium, magnesium, ammonium and alkylammonium; acid addition salts of pharmaceutically acceptable inorganic acids such as hydrochloric, orthophosphoric, sulphuric, phosphoric, nitric, carbonic, boric, sulfamic and hydrobromic acids; or salts of pharmaceutically acceptable organic acids such as acetic, propionic, butyric, tartaric, maleic, hydroxymaleic, fumaric, citric, lactic, mucic, gluconic, benzoic, succinic, oxalic, phenylacetic, methanesulphonic, trihalomethanesulphonic, toluenesulphonic, benzenesulphonic, salicylic, sulphanilic, aspartic, glutamic, edetic, stearic, palmitic, oleic, lauric, pantothenic, tannic, ascorbic and valeric acids; or salts or complexes with pharmaceutically acceptable metal ions, including non-toxic alkali metal salts such as sodium and potassium salts, or non-toxic transition metal complexes such as zinc.

In addition, some of the amino acids or peptides may form solvates with water or common organic solvents. Such solvates are encompassed within the scope of the invention.

By “derivative” is meant any salt, hydrate, protected form, ester, amide, active metabolite, analogue, residue or any other compound which is not biologically or otherwise undesirable and induces the desired pharmacological and/or physiological effect. Preferably the derivative is pharmaceutically acceptable.

The term “tautomer” is used in its broadest sense to include compounds which are capable of existing in a state of equilibrium between two isomeric forms. Such compounds may differ in the bond connecting two atoms or groups and the position of these atoms or groups in the compound.

The term “isomer” is used in its broadest sense and includes structural, geometric and stereoisomers. As the amino acids and peptides that may be synthesised by these techniques may have one or more stereogenic centres, the peptide or peptides containing at least one dicarba bridge are capable of existing in enantiomeric/diastereomeric forms.

In one embodiment, an amino acid or peptide is synthesised as an enriched enantiomer or diastereomer, or as a mixture of any ratio of stereoisomers. An amino acid or peptide can also be synthesised as an enriched geometric isomer (e.g. E- or Z-configured alkenes), or as a mixture of any ratio of geometric isomers. It is however preferred that a mixture of structural, geometric and/or stereoisomers is enriched in the preferred isomer.

It will also be appreciated that the peptide or peptides containing a dicarba bridge can be synthesised as an enriched enantiomer or diastereomer, or as a mixture of any ratio of stereoisomers. Where the dicarba bridge of the peptide or peptides is an alkene-containing dicarba bridge, the alkene-containing group of the bridge may be present as a mixture of any ratio of geometric isomers (e.g. E- or Z-configured alkenes), or as an enriched geometric isomer. Preferably, the alkene-containing dicarba bridge of the peptide or peptides is enriched in the preferred isomer.

In its broadest sense “enriched” means that the mixture contains more of the preferred isomer than of the other isomer. Preferably, an enriched mixture comprises greater than 50% of the preferred isomer, where the preferred isomer gives the desired level of potency and selectivity. More preferably, an enriched mixture comprises at least 60%, 70%, 80%, 90%, 95%, 97.5% or 99% of the preferred isomer. The product which is enriched in the preferred isomer can either be obtained via a stereospecific reaction, stereoselective reaction, isomeric enrichment via separation processes, or a combination of all three approaches.

It must be noted that, as used in the specification, the singular forms “a”, “an” and “the” include plural aspects unless the context clearly dictates otherwise. Thus, for example, reference to “a dicarba bridge” includes a single dicarba bridge, as well as two or more dicarba bridges, and so forth.

Dicarba Bridge

The method of the present invention relates to the preparation of a peptide or peptides containing a dicarba bridge. The dicarba bridge may be unsaturated (for example, the dicarba bridge may contain —CH═CH—, or —C≡C—) or saturated (the bridge may contain —CH₂—CH₂—). It will be appreciated that the peptide or peptides may include one or more additional dicarba bridges.

The dicarba bridge may be formed between two separate peptide chains to form an inter-chain dicarba bridge, or it may form a bridge between two points in a single peptide chain so as to form an intra-chain dicarba bridge, otherwise known as a ring.

In some instances it may be difficult to form dicarba bridges due to steric hinderance, aggregation and/or the need to bring the reactable (metathesisable) groups together. We have found that the use of alternating solid phase peptide synthesis and catalysis enables dicarba bridges to be formed, often in a more efficient manner. The approach of alternating solid phase peptide synthesis and catalysis may be combined with other features which are described herein, such as microwave reaction conditions and the use of turn-inducing groups to enhance the catalysis steps.

The term “dicarba bridge” is used broadly, unless the context indicates otherwise, to refer to a bridging group that includes at least one of the groups selected from —C—C—, —C═C— and —C≡C—. This means that the dicarba bridge could be wholly or partly composed of the groups —C—C—, —C═C— and —C≡C—, or could for example be one of the dicarba bridges shown in formula (I) to (VI) below. In a preferred embodiment, the atoms directly attached to the carbon atoms of the dicarba bridge are carbon or H, or N, S, O or P. More preferably the atoms directly attached to the carbon atoms of the dicarba bridge are C or H. Further or alternative reactions may be performed to introduce substituents other than hydrogen onto the carbon atoms of the dicarba sequence or at other positions on the dicarba bridge.

The term “unsaturated dicarba bridge” is used to refer to dicarba bridges which contain the group —C═C— (referred to as an alkene-containing dicarba bridge) or dicarba bridges which contain the group —C≡C— (referred to as an alkyne-containing dicarba bridge). The term “unsaturated hydrogen dicarba bridge” is used to refer to —CH═CH—.

The term “saturated dicarba bridge” is used broadly, unless the context indicates otherwise, to refer to a bridging group that includes at least a saturated alkane containing dicarba bridge (—C—C—). The term “unsaturated hydrogen dicarba bridge” is used to refer to —CH═CH—.

The term “alkyne-containing dicarba bridge” is used broadly, unless the context indicates otherwise, to refer to a bridging group that includes at least an alkyne group (—C≡C—). This means that the alkyne-containing dicarba bridge could be wholly or partly composed of the group —C≡C—, or could for example be one of the dicarba bridges shown in formula (I) or (II) below.

In addition to the alkyne group, the alkyne-containing dicarba bridge may include any other series of atoms, typically selected from C, N, O, S, and P. The atoms directly attached to the carbon atoms of the alkyne-containing dicarba bridge are preferably carbon. However, any of the other atoms listed above may also be present, with the proviso that the nitrogen atoms present in the compound during metathesis are not free amines (protected amines, such as carbamates and salts, are acceptable). The alkyne-containing dicarba bridge encompasses the following possible bridges, as illustrative examples:

wherein R₁ to R₆ are each independently absent or selected from a divalent linking group. R₁ to R₆ may be the same (for example R₁═R₂) or different (for example R₁≠R₂), or one or more or all of R₁ to R₆ may be absent. Such divalent linking groups should not be groups that poison the metathesis catalyst. Preferably, the divalent linking groups R₁ to R₆ are substituted or unsubstituted alkylene or substituted or unsubstituted alkoxy groups.

The term “alkylene” refers to divalent alkyl groups including straight chain and branched alkylene groups having from 1 to about 20 carbon atoms. Typically, the alkylene groups have from 1 to 15 carbons or, in some embodiments, from 1 to 8, 1 to 6, or 1 to 4 carbon atoms. Examples of straight chain alkylene groups include methyl, ethyl, n-propyl, n-butyl, n-pentyl, n-hexyl, n-heptyl, and n-octyl groups. Examples of branched alkyl groups include, but are not limited to, isopropyl, iso-butyl, sec-butyl, tert-butyl, neopentyl, isopentyl, and 2,2-dimethylpropyl groups. The alkylene groups may also be substituted and may include one or more substituents.

The term “alkoxyl” refers to the divalent group —OR— where R is an alkylene group as defined above. Examples of straight chain alkoxy groups include methoxyl, ethoxyl, propoxyl and longer chain variants. Examples of branched alkoxyl groups include, but are not limited to α-methylmethoxyl and α-methylmethoxyl groups. The alkoxyl groups may also be substituted and may include one or more substituents, which are as defined below.

A “substituted” alkylene or alkoxy group has one or more of its hydrogen atoms replaced by non-hydrogen or non-carbon atoms. Substituted groups also include groups in which one or more bonds to a carbon(s) or hydrogen(s) atom are replaced by one or more bonds, including double or triple bonds which may optionally be blocked, via an adjacent heteroatom. Thus, a substituted group will be substituted with one or more substituents, unless otherwise specified. In some embodiments, a substituted group is substituted (in protected or unprotected form) with 1, 2, 3, 4, 5, or 6 substituents. Examples of substituent groups include halogens (i.e., F, Cl, Br, and I); hydroxyls; alkoxyl, alkenoxyl, alkynoxyl, aryloxyl, aralkyloxyl, heterocyclyloxyl, and heterocyclylalkoxyl groups; carbonyls (oxo); carboxyls; esters; ethers, urethanes; oximes; hydroxylamines; alkoxyamines; aralkoxyamines; thiols; sufides; sulfoxides; sulfones; sulfonyls; sulfonamides; amines; N-oxides; hydrazines; hydrazides; hydrazones; azides; amides; ureas; amidines; guanidines; enamines; imides; isocyanates; isothiocyanates; cyanates; thiocyanates; imines; nitriles (i.e. CN); and the like. Such substituents should not be groups that poison the metathesis catalyst or affect its selectivity.

The term “alkene-containing dicarba bridge” is used broadly, unless the context indicates otherwise, to refer to a bridging group that includes at least an unsaturated alkene (—C═C—). This means that the dicarba bridge could be wholly or partly composed of the group —C═C—, or could for example be one of the dicarba bridges shown in formula (III) or (IV) below. The alkene-containing dicarba bridge (—C═C—) may possess cis- or trans-geometry.

In a preferred embodiment, the atoms directly attached to the carbon atoms of the alkene-containing dicarba bridge are typically H or C, or N, S, O or P. More preferably, the atoms directly attached to the carbon atoms of the alkene-containing dicarba bridge are C or H. Further or alternative reactions may be performed to introduce substituents other than hydrogen onto the carbon atoms of the dicarba sequence or at other positions on the dicarba bridge.

In addition to the C═C dicarba sequence, the dicarba bridge may include any other series of atoms, typically selected from C, N, O, S, and P, although the atoms to either side of the dicarba sequence are preferably carbon, and with the proviso that the nitrogen atoms present in the compound during metathesis are not free amines (protected amines, such as carbamates, and salts are acceptable). Thus, the dicarba bridge encompasses the following possible bridges, as illustrative examples:

wherein R₁ to R₆ are each independently absent or selected from a divalent linking group. R₁ to R₆ may be the same (for example R₁═R₂) or different (for example R₁≠R₂), or one or more or all of R₁ to R₆ may be absent. Such divalent linking groups should not be groups that poison the metathesis catalyst. Preferably, the divalent linking groups R₁ and R₂ are substituted or unsubstituted alkylene or substituted or unsubstituted alkoxyl group, as defined above.

The term “saturated dicarba bridge” or “alkane-containing dicarba bridge” is used broadly, unless the context indicates otherwise, to refer to a bridging group that includes at least a saturated alkane containing dicarba bridge (—C—C—). This means that the dicarba bridge could be wholly or partly composed of the groups —C—C—, or could for example be one of the dicarba bridges shown in formula (V) or (VI) below.

In a preferred embodiment, the atoms directly attached to the carbon atoms of the saturated dicarba bridge are typically H or C, or N, S, O or P. More preferably, the atoms directly attached to the carbon atoms of the saturated dicarba bridge are H or C. Further or alternative reactions can be performed to introduce substituents other than hydrogen onto the carbon atoms of the dicarba sequence or at other positions on the dicarba bridge.

The term “saturated hydrogen dicarba bridge” refer to dicarba bridges which contain, the group —CH₂—CH₂—. Typically, saturated dicarba bridges are prepared by hydrogenation or other reduction of unsaturated dicarba bridges.

In addition to the dicarba sequence, the dicarba bridge may include any other series of atoms. Preferably the other atoms are selected from C, N, O, S, and P, provided that nitrogen atoms present in the compound during metathesis are not free amines (protected amines, such as carbamates and salts, are acceptable). More preferably, the atoms to either side of the dicarba sequence are carbon. Thus, the dicarba bridge encompasses the following possible bridges, as illustrative examples:

wherein R₁ to R₆ are each independently absent or selected from a divalent linking group. R₁ to R₆ may be the same (for example R₁═R₂) or different (for example R₁≠R₂) or one or more or all of R₁ to R₆ may be absent. Such divalent linking groups should not be groups that poison the metathesis catalyst. Preferably, the divalent linking groups R₁ and R₂ are substituted or unsubstituted alkylene or substituted or unsubstituted alkoxyl group, as defined above.

Where the terms “alkyne-containing”, “alkene-containing” or “alkane-containing” or “saturated” are not specified, the term “dicarba bridge” is taken to refer to a bridging group that includes at least one of the groups selected from a saturated dicarba bridge (—C—C—), an unsaturated alkene-containing dicarba bridge (—C═C—) and an alkyne-containing dicarba bridge (—C≡C—) as described above. This means that the dicarba bridge could be wholly or partly composed of the groups —C—C—, —C═C— or —C≡C—, or could for example be any one of the dicarba bridges shown in formula (I) to (VI) above.

Catalysis

The term “catalysis” is intended to encompass steps of the process in which a catalyst is used to achieve a transformation. It will therefore be appreciated that any reference to catalysis could, for example, include the step of subjecting the reactable peptide or reactable peptides to either alkene or alkyne metathesis, or may also include the step of reducing at least one unsaturated dicarba bridge to form an alkene-containing dicarba bridge or a saturated dicarba bridge.

The method of the present invention involves alternating peptide synthesis and catalysis steps to form a peptide or peptides containing an unsaturated dicarba bridge. The steps of catalysis involve metathesis in order to form the dicarba bridge, but may optionally involve reduction of the newly formed dicarba bridge before the step of adding one or more further amino acids to one or both ends of the at least one reactable peptide. When the catalysis includes reduction of the newly formed dicarba bridge, the reduction may occur after the step of adding one or more further amino acids to either end of at least one reactable peptide.

Metathesis

Metathesis is a powerful synthetic tool that enables the synthesis of carbon-carbon bonds via a transition metal-catalysed transformation of alkyl-unsaturated reactants. The construction of dicarba analogues of complex peptides, however, presents more of a synthetic challenge.

The use of uniform metathesis substrates leads to a statistical product distribution and therefore metathesis selectivity is severely compromised. For example, homodimerisation of equivalent olefins A and B in the absence of selectivity results in a statistical mixture of three products (as shown below). The yield of desired products (A-A and B-B) is not more than 50% in the absence of selection. In order to exclusively form the target A-B product, selective metathesis strategies must be employed to avoid the formation of the A-A and B-B homodimers.

Cross-metathesis (CM) is a type of metathesis reaction involving the formation of a new bond across two unblocked, reactive metathesisable groups, to form a new bridge between the two reactive metathesisable groups. For example, using cross-metathesis, a dicarba analogue of insulin having an intermolecular bridge results from formation of a dicarba bridge between two reactive metathesisable groups each located in different chains of insulin.

Ring-closing metathesis (RCM) is a type of metathesis reaction where the two reactive metathesisable groups are located within one peptide chain so as to form an intramolecular bridge, or ring. For example, ring-closing metathesis involves the formation of a dicarba bridge between two reactable metathesisable groups located on a single peptide chain to produce a dicarba analogue of a peptide having an intrachain bridge.

It is preferred that at least one reactable peptide is provided on a solid support. The types of solid supports that may be used are described below.

The use of a solid support provides a number of advantages. Firstly, the combination of peptide synthesis and catalysis using a single solid support is highly efficient. In addition, the catalysts used may be homogeneous catalysts, such as those used to affect metathesis and hydrogenation. The catalysts can be exposed to a resin bound peptide and simply separated from the product peptide via filtration of the resin-peptide from the reaction solution. This eliminates and/or minimises metal-contamination of the product and aids the separation of the product peptide from solution phase by-products and/or impurities. Furthermore, protecting groups for reactive sidechains which are commonly employed in SPPS protocols are also tolerated by organotransition metal catalysts and hence catalysis can conveniently be performed immediately after SPPS.

Tethering a peptide sequence to a solid support can also promote RCM: A pseudo-dilution effect operates on resin to promote RCM over otherwise competing CM reactions. Hence high dilution is not required for the promotion of RCM conversion.

Alkyne Metathesis

Alkyne metathesis can be used to install one or more dicarba bridges in a peptide or peptides. Alkyne metathesis involves the formation of a new alkyne bond from two unblocked or reactive alkynes. The new alkynyl bridge covalently joins the two reactive starting alkynes. As shown below, the alkyne-containing dicarba bridge may be formed between two complementary alkyne-containing metathesisable groups. Cross-metathesis occurs when the alkyne-containing dicarba bridge is formed between two or more peptides having between them two or more complementary alkyne-containing metathesisable groups. For example, where the reactable peptides have at least two metathesisable groups between them (one metathesisable group on each peptide), at least one dicarba bridge formed is an intermolecular dicarba bridge. In this case, the alkyne-containing dicarba bridge is an intermolecular dicarba bridge (shown as (A) below). Alternatively, ring closing metathesis occurs when the alkyne-containing dicarba bridge is formed between two or more complementary alkyne-containing metathesisable groups within a single reactable peptide. For example, where the reactable peptide has at least two complementary metathesisable groups, at least one dicarba bridge formed is an intramolecular bridge. In this case, the alkyne-containing dicarba bridge is an intramolecular dicarba bridge (shown as B below).

It can be difficult to form intramolecular bridges due to aggregation, deleterious hydrogen bonding, and the need to bring the reactable (metathesisable) groups together. The use of microwave radiation and/or the use of turn-inducing groups in the metathesis step (as discussed below) facilitates the metathesis reaction to occur, or occur more efficiently.

In one embodiment of the method of the present invention, the alkyne-containing dicarba bridge that is formed between two amino acids can subsequently be subjected to stereoselective reduction or hydrogenation to preferentially generate either the cis- or the trans isomer of the alkene-containing dicarba bridge. This approach produces a product which is enriched in one of the geometric isomers of the alkene-containing dicarba bridge (perhaps the most/only active isomer). Advantageously, the need for time-consuming chromatographic separation of the unwanted geometric isomer can in some instances be reduced. This optional reduction may be performed after the metathesis step of the method of the present invention, and either before or after the step of adding one or more further amino acids to either end of at least one reactable peptide.

Alkyne metathesis is facilitated by a catalyst. Some metal-complexes are highly active alkyne metathesis catalysts and some are capable of highly selective alkyne metathesis in the presence of other potentially reactable groups, e.g. alkenes.

Catalysts which may be used to perform alkyne metathesis in the method of the present invention are those catalysts which are selective for alkyne-containing metathesisable groups, while not interfering with the functional groups present in the amino acids and peptides between which the alkyne-containing dicarba bridge is formed. Examples of suitable catalysts include those described in Fürstner, A.; Davies, P. W. Chem. Commun. 2005, 2307-2320; Zhang, W.; Moore, Jeffrey S. Adv. Synth. Catal. 2007, 349, 93-120; Grela, K., lgnatowska, J., Organic Letters, 1992, 4(21), 3747; and Mortreux, A.; Coutelier, O. J. Mol. Catal. A: Chem. 2006, 254, 96-104, incorporated herein by reference. There are many catalysts available to achieve this transformation, which vary in their ability to catalyse the metathesis reaction, in their ability to tolerate other functional groups, and their stability towards water and other functional groups. Preferably the catalyst used for alkyne metathesis is a homogenous catalyst. More preferably, the catalyst is a tungsten containing catalyst, or a molybdenum-containing catalyst such as those based on W(IV) and Mo(CO)₆/phenol systems. Still more preferably, the catalyst is a tungsten-containing catalyst. An example of a suitable tungsten-containing catalysts is tris(tert-butoxy)(2,2-dimethylpropylidyne)tungsten.

A preferred tungsten-containing alkyne-metathesis catalyst is a tungsten-alkylidyne complex commonly known as Schrock's catalyst, tris(tert-butoxy)(2,2-dimethylpropylidyne) tungsten(VI), which is shown below. This catalyst is a highly air and moisture sensitive molecule which necessitates the rigorous use of inert and anhydrous reaction atmosphere and solvents respectively. It is however, highly tolerant of a wide range of functionality which is often found in peptide sequences.

The method for the formation of an alkyne-containing dicarba bridge involves the use of complementary pairs of alkyne-containing metathesisable groups connected to an amino acid or connected to an amino acid in a peptide. A metathesisable group is a functional group that can undergo metathesis when unblocked or in an activated state. The alkyne containing metathesisable group may be connected to an amino acid via the amino acid side chain or via the amino group of the amino acid. As an example, a side chain of the amino acid may include at least an alkyne-containing metathesisable group, and the side chain may be wholly or partly composed of the group —C≡C—.

The term “alkyne-containing metathesisable group” is used broadly, unless the context indicates otherwise, to refer to a group that includes at least an alkyne moiety. The alkyne-containing metathesisable group could for example be an alkyne-containing metathesisable group of the general formula drawn below:

The integer n may be 0, 1, 2, 3, 4, 5, 6, 7, 8, 9 or 10. The groups R₇ and R₈ should not be a group that poisons the metathesis catalyst. Preferably, the group R₇ is substituted or unsubstituted alkyl. The group R₈ is either H or substituted or unsubstituted alkyl. Preferably, the group R₈ is either H or methyl.

The term “alkyl” refers to a monovalent alkyl group including straight chain and branched alkyl groups having from 1 to about 20 carbon atoms. Typically, the alkyl group has from 1 to 15 carbons or, in some embodiments, from 1 to 8, 1 to 6, or 1 to 4 carbon atoms. Examples of straight chain alkyl groups include methyl, ethyl, n-propyl, n-butyl, n-pentyl, n-hexyl, n-heptyl, and n-octyl groups. Examples of branched alkyl groups include, but are not limited to, isopropyl, iso-butyl, sec-butyl, tert-butyl, neopentyl, isopentyl, and 2,2-dimethylpropyl groups. The alkyl group may also be substituted and may include one or more substituents. The term “substituted” is as defined above in relation to alkylene groups.

The alkyne-containing metathesisable group may be connected to an amino acid of the reactable peptide or peptides. The alkyne-containing metathesisable group is preferably located on the amino group or on the side chain of the amino acid.

During the metathesis reaction, a by-product is produced, which comprises an alkyne bond which is substituted with the group R₇. Preferably, the groups R₇ are such that the resulting by-product is gaseous, and is eliminated from the reaction mixture. For example, when R₇ is methyl, the by-product is 2-butyne, which evaporates from the reaction mixture to leave the reaction product. Other alkyne by-products, such as 2-pentyne and 3-hexyne (bps 56° C. and 81° C. respectively), could also be generated from the combination of butynylglycine and pentynylglycine residues. These low boiling point liquids are readily removed from the metathesis reaction mixture. It will however be appreciated that techniques for the separation of a non-gaseous by-product from the reaction mixture would also be known by a person skilled in the art.

It is noted that a pair of complementary alkyne-containing metathesisable groups need not be identical. For example, an alkyne-containing metathesisable group in which R₇ is methyl can react with an alkyne-containing metathesisable group in which R₇ is ethyl to form an alkyne-containing dicarba bridge. The term “complementary” is used to indicate that the pair of unblocked alkyne-containing metathesisable groups are not necessarily identical, but are merely complementary in the sense that metathesis can take place between the two alkyne-containing groups.

Alkene Metathesis

Alkene metathesis provides a versatile method for the cleavage and formation of C═C bonds, and involves a mutual intermolecular exchange of alkylidene fragments between two alkene groups.

In alkene metathesis reactions, the redistribution can result in three main outcomes shown below: (A) ring-opening metathesis (ROM) which is sometimes followed by polymerization of the diene (ROMP); (B) ring-closing metathesis (RCM); and (C) cross metathesis. Of particular interest to the present invention are the latter two. ADMET, acyclic diene metathesis, is also an important process.

When a pair of alkene-containing metathesisable groups are incorporated into the primary sequence of a single peptide and subjected to metathesis conditions, an intramolecular reaction will result in the formation of a cyclic peptide (RCM). If however, the pair of alkene-containing metathesisable groups are present within two separate peptide chains, an intermolecular CM reaction will result in the formation of a link between the two peptides. This is shown below.

In one embodiment of the present invention, the alkene-containing dicarba bridge that is formed can subsequently be subjected to reduction to generate the corresponding unsaturated dicarba bridge. This optional reduction may be performed after the metathesis step of the method of the present invention, and either before or after the step of adding one or more further amino acids to either end of at least one reactable peptide.

Alkene metathesis is also facilitated by a catalyst. Catalysts which may be used to perform alkene metathesis in the method of the present invention are those catalysts which are selective for the alkene-containing metathesisable groups, while not interfering with the functional groups present in the amino acids and peptides between which the alkene-containing dicarba bridge is formed. Examples of suitable catalysts include those described in Grubbs, R. N., Vougioukalakis, G. C. Chem. Rev., 2010, 110, 1746-1787, Tiede, S., Berger, A., Schlesiger, D., Rost, D., Lühl, A., Blechert S., Angew. Chem. Int. Ed., 2010, 49, 1-5, and Samojlowicz, C., Bieniek, m., Grela, K. Chem. Rev., 2009, 109, 3708-3742. Preferably, the catalyst used for alkene metathesis is a homogeneous catalyst, such as a ruthenium-based alkene metathesis catalyst.

Many alkene metathesis catalysts are now commercially available or easily synthesised in the laboratory. While early catalysts were poorly defined, lacked functional group tolerance and were highly moisture and oxygen sensitive, later generation catalysts have largely overcome these initial problems. Currently used Ru-alkylidene based catalysts, for example Grubbs' first and second generation catalysts, and the Hoveyda-Grubbs analogues, are robust, display high functional group tolerance and have tunable reactivity under mild experimental conditions. Despite their differing substitution around the core Ru centre, all of the catalysts cycle through an active ruthenium alkylidene species. The variation around the reactive core however, plays an important role in mediating initiation, propagation and substrate specificity.

Where a peptide or peptides is to contain an alkene-containing dicarba bridge and an alkyne-containing dicarba bridge, it is useful to tailor the catalyst used in the metathesis reaction to the substrate in order to achieve regioselective dicarba bridge formation. For example, on exposure to second generation Grubbs' catalyst (a ruthenium alkylidene catalyst bearing phosphine, N-heterocyclic carbene and chloride ligands), a peptide sequence possessing an alkyne and alkene functional group will undergo en-yne metathesis. The same peptide however, exposed to first generation Grubbs' catalyst (a ruthenium alkylidene catalyst bearing only phosphine and chloride ligands), will only undergo alkene cross metathesis.

Preferably, the catalyst is a metal carbene complex such as those shown below. More preferably, where a peptide is to contain at least one alkene-containing dicarba bridge and at least one alkyne-containing dicarba bridge the catalyst used for alkene metathesis is a first generation catalyst.

The formation of an alkene-containing dicarba bridge involves the use of complementary pairs of alkene-containing metathesisable groups connected to an amino acid or connected to an amino acid in a peptide. A metathesisable group is a functional group that can undergo metathesis when unblocked or in an activated state. The alkene-containing metathesisable group may be connected to an amino acid via the amino acid side chain or via the amino group of the amino acid. As an example, a side chain of the amino acid may include at least an alkene-containing metathesisable group, and the side chain may be wholly or partly composed of the group —C═C—.

The term “alkene-containing metathesisable group” is used broadly, unless the context indicates otherwise, to refer to a group that includes at least an alkene moiety. The alkene-containing metathesisable group could for example be an alkene-containing metathesisable group of the general formula drawn below:

The integer n may be 0, 1, 2, 3, 4, 5, 6, 7, 8, 9 or 10. The groups R₉, R₁₀ and R₁₁ should not be a group which poisons the catalyst. Preferably, the groups R₉ and R₁₀ are each independently H or substituted or unsubstituted alkyl as defined above. The group R₁₁ is either H or substituted or unsubstituted alkyl. Preferably, the group R₁₁ is either H or methyl.

The alkene-containing metathesisable group may be connected to an amino acid of the reactable peptide or peptides. The alkene-containing metathesisable group is preferably located on the amino group or on the side chain of the amino acid.

The alkene-containing metathesisable group could for example include the alkene-containing metathesisable groups:—allylglycine (A), crotylglycine (B), prenylglycine (C) and the extended acrylate (D) (as drawn below).

The reactivity of alkenes towards homodimerisation during metathesis, has been categorised into four classes—Type I through IV. Type I alkenes are the most reactive, and are characterised by sterically unhindered and electron-rich olefins such as allyl-(A) and crotylglycine (B). Increasing steric hindrance and decreasing electron density about the olefin, slows homodimerisation. Accordingly these alkenes are categorised in Types II through IV. These include residues such as prenylglycine (C) and the extended acrylate (D). These glycine derivatives are shown below:

During the metathesis reaction, a by-product is produced. Preferably, the by-product is gaseous, and evaporated from the reaction mixture. It will however be appreciated that techniques for the separation of a non-gaseous by-products from the reaction mixture would also be known by a person skilled in the art.

It is noted that a pair of complementary alkene-containing metathesisable groups need not be identical: For example, an allylglycine residue can be metathesised with a crotylglycine residue to generate a new dicarba bridge. The term “complementary” is used to indicate that the pair of unblocked alkene-containing metathesisable groups are not necessarily identical, but are merely complementary in the sense that metathesis can take place between the two alkene-containing groups.

Alternating Peptide Synthesis and Catalysis

In certain peptides it can be difficult to form intramolecular and intermolecular dicarba bridges due to deleterious aggregation and inappropriate positioning of the reactable (metathesisable) groups. The use of microwave radiation and/or the use of turn-inducing groups in the metathesis step (as discussed below) may facilitate the metathesis reaction to occur, or occur more efficiently. However, for many peptide sequences, all of the existing strategies to enhance metathesis, even when used alone or in conjunction, can still fail to produce acceptable dicarba bridge yield.

In this approach, the sequence is grown in a stepwise fashion until both metathesisable residues have been incorporated. One of the peptides may be provided on a solid support. Preferably, the second metathesisable group of the pair is left at or near the N-terminus of the peptide. The resin-supported incomplete sequence is then exposed to the metathesis catalyst to form the dicarba bridge. Following the metathesis step, the resin can either be subjected to secondary catalysis (e.g. hydrogenation or metathesis), or followed immediately with the remaining SPPS to the N-terminus of the desired target peptide. It will be appreciated that this process can be conducted iteratively in order to introduce more than one dicarba bridge. This interrupted approach can be highly successful with sequences which are difficult to metathesise and/or hydrogenate. The scheme below illustrates this approach.

According to one embodiment, there is provided a method for the synthesis of a peptide or peptides containing a dicarba bridge, comprising:

-   (i) synthesising a reactable peptide having at least two     complementary metathesisable groups or one or more reactable     peptides having at least two complementary metathesisable groups     between them; -   (ii) subjecting the reactable peptide or reactable peptides to     metathesis to form a reactable peptide or peptides having at least     one unsaturated dicarba bridge; and -   (iii) adding one or more further amino acids to one or both ends of     at least one of the reactable peptides.

Preferably, the peptide or peptides are synthesised to a point where the required metathesisable group is incorporated. The metathesisable group may be at or near one end of the reactable peptide or peptides. Preferably the metathesisable group is less than 5 residues and most suitably between 0-3 residues from one end of the reactable peptide. When the metathesisable group is 0 residues from one end of the reactable peptide, the metathesisable group is at the end of the reactable peptide.

In one example, where the peptide to be prepared is a dicarba analogue of a naturally occurring peptide or peptides, the method involves the synthesis of a part of the naturally occurring peptide. The part of the peptide or peptides that is synthesised in this step is the part or parts that contain at least two metathesisable groups. The peptide or peptides are then subjected to metathesis to form an unsaturated dicarba bridge. The peptide which is now joined by an unsaturated dicarba bridge is subjected to further peptide synthesis to produce the remainder of the desired peptide or to produce the target truncated peptide or peptides. The one or more further amino acids may be added to one or both ends of the reactable peptide. Where the method involves more than one reactable peptide, and the catalysis forms an intermolecular dicarba bridge, the one or more further amino acids may be added to one or both ends of any of the reactable peptides (e.g. added to one or both ends of either of the peptide chains connected by the intermolecular dicarba bridge). The unsaturated dicarba bridge may optionally be reduced, and the reduction may occur either before or after the step of further peptide synthesis.

Preferably, at least one of the reactable peptides is attached to a solid support.

In an embodiment of the present invention, at least one unsaturated dicarba bridge may be reduced to form an alkene-containing dicarba bridge or a saturated dicarba bridge. The step of reducing the dicarba bridge may occur before or after the step of adding one or more further amino acids to one or both ends of the at least one reactable peptide.

The following is a description of some preferred embodiments of the present invention, with reference to FIGS. 1 to 5. It is therefore intended that these embodiments be considered in all respects as illustrative and not restrictive.

FIG. 1 is a schematic diagram showing a method for preparing a peptide containing a dicarba bridge in which the peptide is synthesised on a solid phase and the method comprises alternating steps of solid phase peptide synthesis (SPPS) and catalysis according to an embodiment of the invention. In this embodiment, the reactable peptide is synthesised having two complementary metathesisable groups. Catalysis includes subjecting the reactable peptide to metathesis to form an unsaturated intramolecular dicarba bridge. Catalysis may also include reduction of the unsaturated intramolecular dicarba bridge (not shown). Following catalysis, one or more further amino acids are added at one end of the reactable peptide.

FIG. 2 is a schematic diagram showing a method for preparing a peptide containing a plurality of dicarba bridges. In this embodiment of the method of the present invention, the peptide is synthesised on a solid phase to contain two complementary metathesisable groups. The reactable peptide is then subjected to catalysis, which includes metathesis to form an unsaturated dicarba bridge. The catalysis may also include reduction of the unsaturated intramolecular dicarba bridge (not shown). Following catalysis, one or more further amino acids are added at one end of the reactable peptide, with at least two of those amino acids containing metathesisable groups. These metathesisable groups are subjected to catalysis, which includes metathesis to form an unsaturated dicarba bridge (not shown). The catalysis may also include reduction of the unsaturated intramolecular dicarba bridge. The steps of peptide synthesis and catalysis is continued until a peptide having “x” dicarba bridges is produced.

FIG. 3 is a schematic diagram showing a method for ligating several peptides together. In this embodiment, a first peptide is synthesised on a solid support having at least one metathesisable group. In the catalysis step, a second peptide having at least one metathesisable group is provided and the two peptides are subjected to catalysis, which includes metathesis to form an unsaturated dicarba bridge. The catalysis may also include reduction of the unsaturated intramolecular dicarba bridge (not shown). Following catalysis, one or more further amino acids are added at one end of the first peptide, with at least one of those amino acids containing a metathesisable group. Alternating steps of catalysis (e.g. metathesis and optionally reduction) and solid phase peptide synthesis (SPPS) is confirmed until a peptide having “x” dicarba bridges is produced according to an embodiment of the invention.

FIG. 4 is a schematic diagram showing a method for forming intermolecular and intramolecular dicarba bridges in which a first peptide is synthesised to contain at least one metathesisable group. In the catalysis step, a second peptide having at least one metathesisable group is provided and the two peptides are subjected to catalysis, which includes metathesis to form an unsaturated dicarba bridge. The catalysis may also include reduction of the unsaturated intramolecular dicarba bridge (not shown). Following catalysis, one or more further amino acids are added at one end of the first peptide, with at least two of those amino acids containing metathesisable groups. These metathesisable groups are subjected to catalysis, which includes metathesis to form an unsaturated dicarba bridge. The catalysis may also include reduction of the unsaturated intramolecular dicarba bridge (not shown). The resulting peptide contains one intramolecular and one intermolecular dicarba bridge.

FIG. 5 is a schematic diagram showing a method for preparing a first peptide containing an interchain dicarba bridge. In this embodiment of the method of the present invention, a peptide is synthesised on a solid phase to contain at least one metathesisable group. In the catalysis step, a second peptide having at least one metathesisable group is provided and the two peptides are subjected to catalysis, which includes metathesis to form an unsaturated dicarba bridge. The catalysis may also include reduction of the unsaturated intramolecular dicarba bridge (not shown). Following catalysis, one or more further amino acids are added at the end of the second reactable peptide.

When a reactable peptide having an intramolecular bridge is desired, at least two complementary metathesisable groups are provided on a single peptide. Metathesis is conducted to form the dicarba bridge and then one or more further amino acids is added to one or both ends of at least one reactable peptide.

When a reactable peptide having an intermolecular bridge is desired, at least two reactable peptides have at least two complementary metathesisable groups between them. Metathesis is conducted to form the dicarba bridge between at least two complementary metathesisable groups forming a bridge between two reactable peptides, and then one or more further amino acids are added to one or both ends of at least one of the reactable peptides.

For the synthesis of a peptide with multiple dicarba bridges, which may be either intramolecular, intermolecular or a combination of both, the method comprises:

-   (i) providing a reactable peptide or peptides comprising a series of     amino acids, wherein one (for CM/CAM) or two (for RCM/RCAM)     metathesisable groups is/are included within the sequence, -   (ii) subjecting the peptide of peptides to metathesis to form a     peptide having at least one dicarba bridge, -   (iii) adding one or more further amino acids comprising at least two     complementary metathesisable groups to one or both ends of at least     one reactable peptide; and -   (iv) repeating steps (ii) and (iii) at least once.

Microwave Reaction Conditions

It is possible to perform the metathesis reaction under microwave reaction conditions. This may assist the metathesis in addition to the advantages provided by the method. For instance, when the metathesisable groups are unblocked, but the arrangement, length or spatial orientation of the reactable organic compound prevents the metathesisable groups from being close enough to one another to enable the reaction to proceed. An alternative strategy is described below (see the description of “turn-inducing groups” below).

The microwave reaction conditions involve applying microwave radiation to the reactable peptide (preferably attached to a solid support) in the presence of the metathesis catalyst for at least part of the reaction, usually for the duration of the reaction. The microwave or microwave reactor may be of any type known in the art, operated at any suitable frequency. Typical frequencies in commercially available microwave reactors are 2.45 GHz, at a power of up to 500 W, usually of up to 300 W. The temperature of the reaction is preferably at elevated temperature, as a consequence of the microwave radiation, preferably at reflux, or around 100° C. The reaction is preferably performed in a period of not more than 5 hours, suitably for up to about 2 hours.

Turn-Inducing Groups

Another strategy which may further improve the performance of a metathesis reaction (in particular, ring closing metathesis) between two complementary metathesisable groups (alkenes or alkynes) is the use of turn-inducing groups. This strategy is particularly useful for ring-closing metathesis where the metathesisable groups are located within a single peptide. As described above, this strategy can also be used in combination with microwave irradiation.

According to this embodiment, a reactable peptide is synthesised to contain a pair of unblocked complementary metathesisable groups, and a turn-inducing group located between the pair of complementary metathesisable groups. The turn-inducing group bends the backbone of the peptide for metathesis to form a dicarba bridge. Following metathesis, and optionally reduction of the unsaturated dicarba bridge, one or more further amino acids are added to either end of the reactable peptide.

The peptide backbone in α-peptides is generally linear as the component amino acids (especially when these are the 20 common amino acids, with exception of proline) form trans-configuration peptide bonds. Proline, a pyrrolidine analogue, can induce a turn in an otherwise linear peptide. This is a naturally-occurring turn-inducing group. This embodiment is particularly suited to those peptides that do not contain a naturally-occurring turn-inducing amino acid, such as proline.

In alkyne metathesis, the linear geometry of the alkyne (—C≡C—) functional group may be detrimental to achieving high yielding metathesis. The new alkyne bond (—C≡C—) that is formed will be linear, and the alkyne-containing metathesisable groups need to be brought in close proximity in order to react. Particularly, in ring-closing alkyne metathesis (ROAM), this may require that the backbone of the peptide curve around to bring the two alkyne-containing metathesisable groups into such a conformation. In some instances this can be conformationally disfavoured and therefore alkyne metathesis may be relatively low yielding or not occur. Accordingly, it may be preferable to include a turn-inducing group between a pair of complementary alkyne-containing metathesisable groups or in a position adjacent to an alkyne-containing metathesisable group so that the conformation of the peptide backbone can be ‘unnaturally’ altered to allow a pair of complementary alkyne-containing metathesisable groups to be brought into an improved conformation for alkyne metathesis.

Preferably the turn-inducing group is a turn-inducing amino acid, dipeptide or protein, and is preferably synthetic (non-naturally occurring). Examples of suitable synthetic turn-inducing amino acids are the pseudoprolines, including derivatives of serine, threonine and cysteine (shown below). The pseudoprolines have been derivatised to contain a cyclic group between the amino acid sidechain (via the —OH or —SH group), and the amino nitrogen atom. A typical derivatising agent is CH₃—C(═O)—CH₃, such that the turn-inducing amino acids are:

These turn-inducing residues are often prepared as dipeptide units to aid incorporation into peptides. An example of a suitable turn-inducing residue is 5,5-dimethylproline which is stable and may stay in the peptide permanently. However, after metathesis, some pseudoproline(s) may be converted back to the underivatised amino acid (serine, threonine or cysteine) by removal of the derivatising agent usually on treatment with acid. The conditions for cleavage of the peptide from a solid support will usually achieve this.

If, for example, the turn-inducing amino acid is one of pseudo-serine, pseudo-proline or pseudo-cysteine, then the method may further comprise the step of converting the pseudo-serine, pseudo-proline or pseudo-cysteine to serine, proline or cysteine, respectively.

The use of pseudoproline residues can be combined with the other preferred features described herein. As one example, pseudo-proline residues can be used in combination with microwave conditions.

As described above, a turn inducing residue is provided between the two complementary metathesisable groups of the reactable peptide which will form the dicarba bridge, in order to bring them closer together during the metathesis step. However, it will be appreciated that where more than one dicarba bridge is to be formed, the one or more further amino acids that are added at either end of the reactable peptide after the metathesis step may also include pseudoproline residue between the two metathesisable groups which are added during the peptide synthesis step to form the further dicarba bridge or bridges.

Tethers Between Peptide Sequences

In some instances, cross-metathesis between peptide sequences can be difficult and low yielding. Success is often sequence dependent and relies on favourable positioning of reacting motifs which can be hampered by peptide size, aggregation, deleterious hydrogen bonding/salt bridges and steric constraints imposed by the primary sequence.

One approach by which we can enhance the metathesis between two complementary metathesisable groups (alkenes or alkynes) is to utilise a contiguous peptide sequence, containing the two amino acids or peptides to be connected by a dicarba bridge, joined together via a removeable tether. Such an approach capitalises on the improved positioning of the reactive motifs imposed by the tether and hence exploits the enhanced reactivity via an intramolecular reaction (RCM) compared to an intermolecular reaction (CM) to produce superior ligation yields. Such an approach is illustrated below:

In this example, SPPS is used to generate a single peptide sequence where a transient/removeable tether is positioned between the two metathesisable groups. Catalysis is then performed on the resin-bound peptide (RCM, RCAM and/or H) and the resultant cyclic peptide is then cleaved open at the tether to result in the target acyclic peptide. The final peptide is analogous to that produced via a direct CM reaction between two peptide sequences. The resin-appended sequence can then be further elaborated via SPPS in a number of positions as shown above.

Groups which may function as a removeable tether are structurally diverse. The removeable tether may be any motif which can be chemoselectively incorporated and removed from the sequence, either chemically or enzymatically. The removeable tether may be a motif which can be added by reductive amination. The removeable tether may be a motif which can be removed by photolysis. Preferably, the removeable tether is a motif which also promotes a turn in the backbone of the primary sequence (similarly for the turn-inducing residues described above). In this approach, the metathesis reaction may be enhanced by suitable positioning of the reactive motifs. As one example, the removeable tether may be hydroxy-6-nitrobenzaldehyde.

Reduction

In some instances, the dicarba bridge may need to assume a particular conformation in order to serve as a suitable peptidomimetic. It may therefore be advantageous for the dicarba bridge to adopt a particular geometry. The conformation of the dicarba bridge will change if the dicarba bridge is saturated compared to an unsaturated dicarba bridge. This may affect the structure and/or activity of the resultant peptide.

The product of the alkyne or alkene metathesis is a dicarba analogue of insulin with a new unsaturated alkyne- or alkene-containing dicarba bridge (—C≡C— or —C═C—). If the target compound is to contain an alkane-containing dicarba bridge (—CH₂—CH₂—), the process may further comprise the step of subjecting the alkyne/alkene-containing dicarba bridge to complete reduction. If the target compound is to contain an alkene-containing dicarba bridge (—CH═CH—), the process may further comprise the step of subjecting the alkyne-containing dicarba bridge to semi-reduction.

Hydrogenation of an Alkyne- or Alkene-Containing Dicarba Bridge

The product of the alkyne or alkene metathesis is a dicarba analogue of insulin with a new unsaturated alkyne- or alkene-containing dicarba bridge (—C≡C— or —C═C—). If the target compound is to contain an alkene-containing dicarba bridge (—C═C—) or an alkane-containing dicarba bridge (—C—C—) the process may further comprise the step of reducing (e.g. via hydrogenation) the alkyne or alkene bond.

The hydrogenation can be conducted at any temperature, such as room temperature or at elevated temperature. The reaction is typically conducted at elevated pressure, although if slower reaction times can be tolerated, the reaction can be performed at atmospheric pressure. The hydrogenation reaction can be performed on substrates which are attached or unattached to a solid support.

Hydrogenation of the unsaturated dicarba bridge can be performed with any known hydrogenation catalyst. Examples of suitable catalysts include those described in March, J. Advanced Organic Chemistry: Reactions, Mechanisms and Structure. 1992, pages 771 to 780 and in Ojima, I. Catalytic Asymmetric Synthesis; Wiley-VCH: New York, 2000; Second Edition, Chapter 1, 1-110, incorporated herein by reference. Suitable hydrogenation catalysts are chemoselective for unblocked, non-conjugated carbon-carbon double or triple bonds.

Suitable hydrogenation catalysts may be either insoluble in the reaction medium (heterogeneous catalysts) or soluble in the reaction medium (homogeneous catalysts). Examples of suitable heterogeneous catalysts include Raney nickel, palladium-on-charcoal (Pd/C) and platinum oxide. Examples of suitable homogeneous catalysts include Wilkinson's catalyst, other Rh(I) phosphine complexes, and Ru(II) phosphine complexes.

The particular hydrogenation catalyst that is used will depend on the target compound. For example, if the target compound is to include a saturated alkane-containing dicarba bridge, a hydrogenation catalyst capable of reducing an alkyne bond (possibly via an alkene intermediate) or an alkene bond to an alkane bond will be selected (“complete hydrogenation”). As another example, if the target compound is to include an unsaturated alkene-containing dicarba bridge, a hydrogenation catalyst capable of reducing an alkyne bond to an alkene bond (“semi-hydrogenation”) will be selected.

If the target compound is to include a saturated alkane-containing dicarba bridge, the hydrogenation is performed with a catalyst that is chemoselective for unblocked, non-conjugated carbon-carbon triple and carbon-carbon double bonds as distinct from other double bonds such as carbon-oxygen double bonds in carbonyl groups, carboxylic acids and blocked conjugated double bonds. The hydrogenation of an alkyne (C═C) bridge to an alkane (CH₂—CH₂) bridge involves the initial step of producing an alkene (C═O) bridge. The alkene bridge then becomes a substrate for further reduction to finally produce the required CH₂—CH₂ bridge.

Any catalyst which is chemoselective for unblocked non-conjugated carbon-carbon triple and double bonds may be used. Examples of hydrogenation catalysts capable of reducing an alkyne bond to an alkane bond include palladium-on-charcoal (Pd/C), platinum oxide, and Raney nickel. Hydrogenation catalysts which are suitable for reducing an alkyne or alkene bond to an alkane bond also include asymmetric hydrogenation catalysts. Although the use of an asymmetric hydrogenation catalyst is not necessary for the hydrogenation of the alkyne- or alkene-containing dicarba bridges, asymmetric hydrogenation catalysts can nevertheless be used. Any asymmetric hydrogenation catalyst which is chemoselective for unblocked non-conjugated carbon-carbon double or triple bonds may be used. Catalysts in this class are described in U.S. Pat. No. 5,856,525 which is incorporated herein by reference. Such homogenous hydrogenation catalysts are tolerant of sulfide, and disulfide bonds, so that the presence of disulfide bonds and the like will not interfere with the synthetic strategy. Examples of suitable asymmetric hydrogenation catalysts are the chiral phosphine catalysts, including chiral phospholane Rh(I) catalysts.

Some hydrogenation catalysts are chemoselective for alkyne groups as distinct from alkene groups. Such catalysts are thus capable of hydrogenating an alkyne and stopping the reaction at an alkene. The hydrogenation of an alkyne-containing dicarba bridge (—C≡C—) to form an alkene containing dicarba bridge (—C═C—) is therefore performed with a catalyst that is chemoselective for unblocked non-conjugated carbon-carbon triple bonds as distinct from other double bonds such as carbon-carbon double bonds, carbon-oxygen double bonds in carbonyl groups, carboxylic acids and blocked conjugated double bonds. Any catalyst which is chemo selective for unblocked non-conjugated carbon-carbon triple bonds may be used.

Within the group of hydrogenation catalysts that are chemoselective for alkyne groups as distinct from alkene groups, are a group of catalysts which also stereoselectively reduce an alkyne-containing dicarba bridge to form an alkene-containing dicarba bridge that is enriched in either the cis- or the trans-isomer. This method allows biased generation of either the cis or trans-isomer by selecting a catalyst which produces a product enriched in the desired isomer. The controlled reduction of C═C and C≡C in organic compounds is an important synthetic transformation and many catalysts are available to achieve this end. Partial conversion of alkynes into alkenes provides a particularly useful route to geometrically well defined alkenes. A large number of well defined homogeneous transition metal complexes can be used to affect stereoselective semi-hydrogenation of the C≡C bond, and many of these catalysts are also tolerant of a wide range of organic functionality. Organochromium, iron, ruthenium, osmium, rhodium, iridium and palladiaum complexes, inter alia, have all been used in semi-hydrogenation reactions of alkynes. Reaction conditions (e.g. solvent, temperature) play a large role in influencing reaction selectivity. Towards this end, catalysts and reaction conditions can be coupled to selectivity and stereoselectivity perform an alkyne (C≡C) to alkene (C═C) transformation in the presence of existing C═C bonds without resulting in over-reduction. For example, zerovalent palladium catalysts bearing bidentate nitrogen ligands are able to homogeneously hydrogenate alkynes to Z-alkenes and do not reduce existing alkene functionality.

Any catalyst which is stereoselective and chemoselective for unblocked non-conjugated carbon-carbon triple bonds may be used. Catalysts in this class include those described in Kluwer, A. M., Elsevier, C. J. “The Handbook of Homogeneous Hydrogenation”, 2007, Wiley-VCH (de Vries, J. G., Elsevier, C. J. (Editors)), Ch 14—Homogeneous hydrogenation of Alkynes and Dienes, pp 375-411, incorporated herein by reference. Examples of suitable chemoselective hydrogenation catalysts include poisoned Lindlar's catalyst, Pd(0), Ru(II), ruthenium carbonyl clusters, Pt(0), P₂—Ni, chromium tricarbonyl compounds of the generic formula [Cr(CO)₃(arene)], Fe(II) catalyst presursors such as [(PR₃)FeH(N₂)]BPh₄, [(RP₃)FeH(H₂)]BPh₄ and (PR₃═P(CH₂CH₂PPh₂)₃), osmium catalysts such as [OsH(Cl)(CO)(PR₃)₂) and rhodium catalysts such as the Schrock/Osborn cationic Rh-catalyst.

In one example of chemoselective and stereoselective hydrogenation, the alkyne-containing dicarba bridge is hydrogenated in the presence of Pd(0)-catalyst. The alkyne-containing dicarba bridge is enriched in the cis-isomer is generated. In another example performing the hydrogenation in the presence of a Ru(II)-catalyst results in an alkene-containing dicarba bridge enriched in the trans-isomer. This is shown schematically below.

Where the dicarba bridge of the peptide or peptides is an alkene-containing dicarba bridge, the bridge may be present as a mixture of any ratio of geometric isomers (e.g. E- or Z-configured alkenes), or as an enriched geometric isomer. As defined above, “enriched” means that the mixture contains more of the preferred isomer than of the other isomer. Preferably, an enriched mixture comprises greater than 50% of the preferred isomer, where the preferred isomer gives the desired level of potency and selectivity. More preferably, an enriched mixture comprises at least 60%, 70%, 80%, 90%, 95%, 97.5% or 99% of the preferred isomer.

When the product produced by the method of the present invention is a peptide having an alkyne-containing dicarba bridge, the step of hydrogenating the alkyne-containing dicarba bridge can be performed with the peptide attached to a resin. When the peptide substrate is attached to a resin, the hydrogenation step uses a homogeneous hydrogenation catalyst.

The step of hydrogenating the dicarba bridge may occur after the metathesis step and before the addition of one or more further amino acids to the reactable peptide or peptides. Alternatively, the hydrogenation step may occur after the addition of one or more further amino acids to the reactable peptide or peptides.

Preferably, the step of hydrogenating the alkyne-containing dicarba bridge occurs before one or more further amino acids are added to the at least one peptide. This can result in an increased yield for the hydrogenation.

Reduction of an Alkyne-Containing Dicarba Bridge

As described above, alkyne metathesis produces an alkyne-containing dicarba bridge formed between two amino acids. This alkyne-containing dicarba bridge may be converted to the corresponding alkene-containing dicarba bridge by reduction methods other than hydrogenation of the alkyne-containing dicarba bridge.

One such example includes the stereoselective reduction on an alkyne-containing dicarba bridge to an alkene-containing dicarba bridge via hydrosilylation and protodesilylation. The alkene that results is enriched in the trans-isomer. This type of reduction is described in Fürstner, A., Radkowski, K. Chem. Commun. 2002, 2182 and Lacombe, F., Radkowski, K., Seidel, G. and Fürstner, A., Tetrahedron, 2004, 60, 7315, and incorporated herein by reference. In this approach, the alkyne-containing dicarba bridge can be selectively reduced to the trans-isomer by trans-selective hydrosilylation followed by protodesilylation. The two steps involved in this selective reduction are shown below.

The hydrosilylation step may be performed using (EtO)₃SiH in the presence of the cationic ruthenium complex [Cp*Ru(MeCN)₃]PF₆. In this reaction the HSi(OEt)₃ reagent is added across the alkyne bond of the alkyne-containing dicarba bridge with cis-selectively. After protodesilylation, a product that is enriched in the trans-isomer across an alkene-containing dicarba bridge is produced.

When the product produced by the method of the present invention is a peptide having an alkyne-containing dicarba bridge, the step of reducing the alkyne-containing dicarba bridge can be performed with the peptide attached to a resin.

The step of reducing the dicarba bridge may occur after the metathesis step and before the addition of one or more further amino acids to the reactable peptide or peptides. Alternatively, the reduction step may occur after the addition of one or more further amino acids to the reactable peptide or peptides.

Preferably, the step of reducing the alkyne-containing dicarba bridge occurs before one or more further amino acids are added to the at least one peptide. This can result in an increased yield for the reduction.

Regioselective Formation of Multiple Dicarba Bridges

The method for preparing a peptide or peptides containing a dicarba bridge as described above is suitable to form multiple dicarba bridges. The peptide or peptides may include one or more alkyne-containing dicarba bridge, one or more alkene-containing dicarba bridge or one or more alkane-containing dicarba bridge, or any combination thereof.

In one embodiment, the method for preparing a peptide containing a plurality of unsaturated dicarba bridges, comprises:

-   (i) providing a reactable peptide having at least two complementary     metathesisable groups; -   (ii) subjecting the reactable peptide to metathesis to form a     reactable peptide having at least one dicarba bridge; -   (iii) adding one or more further amino acids comprising at least two     complementary metathesisable groups to one or both ends of the at     least one reactable peptide; and -   (iv) repeating steps (ii) and (iii) at least once.

It will be appreciated that at least one unsaturated dicarba bridge may be reduced to form an alkene-containing dicarba bridge or an unsaturated dicarba bridge. The reduction may occur before or after further amino acids are added to the at least one peptide. Alternatively, the reduction could take place once the peptide containing a plurality of unsaturated dicarba bridges is produced. In this embodiment, when the plurality of dicarba bridges are introduced into a single peptide sequence it may be possible to produce a peptide which has some α-helical structure, or a stabilised α-helical structure.

In another approach, where more than two metathesisable groups are present on the peptide or peptides, it may be necessary to include at appropriate locations in the peptide or peptides, one or more blocked complementary metathesisable groups which are or deactivated for the times when different pairs of metathesisable groups are being linked together, and unblocked or “activated” to enable later reaction to occur between those pairs. Accordingly, for each bridge-forming pair, there needs to be an unblocking reaction available that will selectively unblock the required pairs.

The first pair to be subjected to metathesis (alkene metathesis or alkyne metathesis) need not be blocked. The pair of unblocked complementary metathesisable groups is then subjected to the reactions described above to form a dicarba bridge.

Tandem Alkyne Metathesis

When the peptide or peptides are to contain two or more alkyne-containing dicarba bridges, it is important to avoid the formation of an intractable mixture of different products from random metathesis between pairs of alkyne-containing metathesisable groups or between an alkyne-containing metathesisable group or groups and a formed alkyne-containing dicarba bridge.

In one approach, a pair of complementary alkyne-containing metathesisable groups may be introduced during synthesis of a peptide and alkyne metathesis conducted to form a first alkyne-containing dicarba bridge before subsequent pairs of complementary alkyne-containing metathesisable groups are added to one or both ends of the at least one reactable peptide. Blocking groups may also be used in combination, to allow regioselective formation of the particular alkyne containing dicarba bridges and/or to block any formed dicarba bridges from further reacting. It will be appreciated that any combination of these approaches may be used to prepare the desired product.

An example of a typical route for the introduction of two alkyne-containing dicarba bridges by the method of the present invention is shown below:

In example (A) above, two metathesisable groups in a single peptide are subjected to ring-closing alkyne-metathesis (ROAM) to produce an alkyne-containing dicarba bridge. Following this, the alkyne-containing dicarba bridge is blocked and one or more further amino acids, including an alkyne-containing metathesisable group is added to one end of one reactable peptide, and a second alkyne-containing metathesisable group is added to another separate reactable peptide. These two separate peptides could represent two chains of a naturally occurring peptide, or could be two copies of the same peptide. The unblocked alkyne-containing metathesisable groups are then subjected to alkyne-metathesis (CAM) to produce a second alkyne-containing dicarba bridge. The blocked alkyne-containing dicarba bridge may then be unblocked to produce a peptide containing one intramolecular and one intermolecular alkyne-containing dicarba bridge.

In example (B) above, a peptide is synthesised having two unblocked metathesisable groups and one blocked metathesisable group between two reactable peptides. The unblocked metathesisable groups are subjected to ring-closing alkyne-metathesis (ROAM) to produce an alkyne-containing dicarba bridge. Following this, the alkyne-containing dicarba bridge is reduced to an alkene-containing dicarba bridge and the blocked alkyne-containing metathesisable group is unblocked. A second alkyne-containing metathesisable group is added to another separate reactable peptide, and the two peptides are subjected to alkyne-metathesis (CAM) to produce a second alkyne-containing dicarba bridge.

Blocking and Activation for Alkyne Metathesis

For metathesis to occur between two alkynes, the alkynes must not be blocked or protected. A blocking group is any group that prevents metathesis from taking place in the presence of a metathesis catalyst. Preferably, a blocking group is used to prevent reaction of the alkyne metathesisable group during olefin metathesis, where the dicarba bridge containing peptide or peptides are to include both an alkyne-containing dicarba bridge and an alkene-containing dicarba bridge. Blocking groups may also be provided on a formed alkyne-containing bridge. In a preferred embodiment, the blocking group is provided on either the unreacted alkyne-containing metathesisable group or the alkyne-containing dicarba bridge, during olefin metathesis.

Examples of blocking groups for an alkyne containing metathesisable group or an alkyne-containing dicarba bridge include dicobalt hexacarbonyl groups. Removal of one or both of the blocking groups unblocks the alkyne-containing-metathesisable group to enable alkyne metathesis to take place or unblocks the alkyne-containing dicarba bridge. It is noted that for subsequent alkyne metathesis the pair of alkyne-containing metathesisable groups that remain after unblocking need not be identical: For example, after deblocking, but-4-ynylglycine and pent-4-ynylglycine may be metathesised to form a new alkyne bridge. The term “complementary” is used to indicate that the pair of unblocked alkyne-containing metathesisable groups are not necessarily identical, but are merely complementary in the sense that cross-metathesis can take place across the two alkyne groups.

As described above, using a combination of blocking and unblocking mechanisms allows regioselective synthesis of multiple dicarba bridges (intra and/or inter-chain) in dicarba analogues.

Tandem Alkene Metathesis

When the target peptide is to contain two or more alkene-containing dicarba bridges, it is also important to avoid random metathesis occurring between pairs of alkene-containing metathesisable groups or between an alkene-containing metathesisable group and an alkene-containing dicarba bridge.

For alkene metathesis, suitable groups for forming the first pair of complementary metathesisable groups which are not blocked are —CH═CH₂ and —CH═CH—CH₃. These groups may be included in insulin by peptide synthesis, and may be provided via an amino acid connected to —CH═CH₂ or having —CH═CH₂ in its side chain optionally with any divalent linking group linking the carbon at the “open” end (the —CH═ carbon atom) to the amino acid backbone, such as an -alkylene-, -alkylene-carbonyl-, and so forth. Examples of —CH═CH₂-containing amino acids and —CH═CH—CH₃-containing amino acids are allylglycine and crotylglycine, respectively. Each of these amino acids contains the divalent linking group —CH₂— between the alkylene and the amino acid (peptide) backbone.

At the completion of that reaction (and optionally after hydrogenation of the first dicarba bridge), the blocked second pair of complementary metathesisable groups, can be subjected to an unblocking reaction.

When the first pair of complementary metathesisable groups are olefins, suitable functional groups for forming the second pair of complementary metathesisable groups are di-blocked alkylenes, such as the group —CH═CR₁₂R₁₃, in which R₁₂ and R₁₃ are each independently selected from blocking groups, such as alkyl. R₁₂ and R₁₃ are preferably alkyl of C1 to C15. More preferably, R₁₂ and R₁₃ are small chain alkyls, for example methyl. Again, there may be a divalent linking group between the —CH═ carbon atom, and the amino acid backbone, such as an alkylene group, for instance —CH₂—. An example of an amino acid containing this group is prenylglycine, or protected prenylglycine.

The unblocking reaction, or activation reaction, to convert the pair of di-blocked alkylenes into an unblocked alkylenes involves subjecting the blocked second pair of complementary metathesisable groups to cross-metathesis with a disposable olefin, to effect removal of the blocking groups (such as R₁₂ and R₁₃ in the example shown above).

It will be understood that in this case, cross-metathesis is used to replace the group ═CR₁₂R₁₃ with another unblocked group ═CH₂ or ═CHR₁₄, (in which R₁₄ may be —H, functionalised alkyl or alkyl for instance) which is then “activated” or “unblocked” and ready for being subjected to cross-metathesis for the formation of a dicarba bridge, using the same techniques described above.

The conditions for this activation-type of cross-metathesis are the same as described above for the dicarba bridge forming metathesis. It can be performed under microwave conditions, although it need not be, as the disposable olefin is a smaller molecule and less subject to the spatial constraints as larger reactable organic compounds and single reactable organic compounds in which intramolecular bridges are to be formed.

The “disposable olefin” is suitably a mono-substituted ethylene (such as monoalkylated ethylene—such as propene, which is mono-methylated ethylene), or a 1,2-disubstituted ethylene (such as high purity 2-butene, and optionally of cis or trans geometry, or a mixture thereof). Previously, commercial 2-butene has been attempted to be used as the disposable olefin in this unblocking reaction, and the reaction is thus sometimes referred to as “butenolysis”. However, commercially available 2-butene (which is a mixture of cis- and trans-2-butene) can inhibit olefin metathesis due to low level butadiene contaminants.

The substituents on the substituted ethylene disposable olefin are substituents that do not participate in the reaction. Examples are alkyl or a functionalised (substituted) alkyl. The functional group of the functionalised alkyl is suitably a polar functional group, to assist with swelling of the solid support, and solubility. Examples are hydroxyl, alkoxyl, halo, nitrile and carboxylic acids/esters. One specific example is the di-ester functionalised disposable olefin 1,4-diacetoxy-2-butene.

Thus the disposable olefin is suitably a 1,3-butadiene-free disposable olefin, or a 1,3-butadiene-free mixture of disposable olefin and is preferably 1,3-butadiene-free olefin or olefin mixture of one or more of the following olefins:

wherein X and Y are each independently selected from the group consisting of —H, alkyl and alkyl substituted with one or more substituents selected from halo, hydroxyl, alkoxyl, nitrile, acid and ester.

Preferably, at least one of X and Y is not H.

Preferably, in the case of the alkyl substituents, the substituent is preferably on the carbon atom. Preferably the substituted alkyl is a substituted methyl. According to one embodiment, at least one of X and Y is a substituted alkyl, such as a substituted methyl. X and Y may be the same or different. The olefins may be cis or trans, or mixtures of both.

Blocking and Activation for Alkene Metathesis

For metathesis to occur between two alkene groups (olefins), the alkenes must not be blocked by any steric or electronic blocking groups. A steric blocking group is any bulky group that sterically prevents the metathesis from taking place in the presence of a cross-metathesis catalyst. Examples of steric blocking groups on an olefin are alkyl. Prenylglycine is an example of an amino acid containing a dialkyl-blocked olefin side chain (specifically, dimethyl-blocked). Removal of one or both of the blocking groups unblocks the olefin, and enables the cross-metathesis to take place.

It is noted that the pair of metathesisable groups that remain after unblocking need not be identical—a mono-substituted olefin (such as a mono-methylated olefin) and an unsubstituted olefin (being unsubstituted at the open olefinic end) can form a suitable pair of cross-metathesisable groups. The term “complementary” is used to indicate that the pair of unblocked metathesisable groups are not necessarily identical, but are merely complementary in the sense that cross-metathesis can take place across the two olefinic groups.

Electronic blocking refers to the presence of a group on the reactable organic compound or compound that modifies the electronic nature of the olefin group of the reactable organic compound (which would otherwise undergo cross-metathesis), so as to prevent that olefin group from undergoing cross-metathesis. An example of an electronic blocking group is when the double bond is in conjugation with a C═O group—that is, a double bond adjacent to an α,β-unsaturated carbonyl containing group (e.g. C═C—C═C—C═O where the C═C portion is the otherwise reactable group). The α,β-unsaturation withdraws electrons away from the olefinic cross-metathesisable group causing electronic blocking and prevention of cross-metathesis. By using a combination of blocking mechanisms, a series of pairs of metathesisable groups in the reactable peptide or peptides of insulin can be designed, with different reaction conditions to effect selective unblocking of particular pairs. In this way, it becomes possible to regioselectively synthesise multiple dicarba bridges (inter and/or intramolecular) in insulin.

Tandem Alkene and Alkyne Metathesis

The use of tandem alkene and alkyne metathesis to facilitate regioselective synthesis of multiple dicarba bridges is a viable strategy for the synthesis of a peptide or peptides containing at least one alkyne containing dicarba bridge and at least one alkene-containing dicarba bridge. Modern metathesis catalysts are highly chemoselective and are readily tuned to unsaturated substrates to achieve maximum selectivity.

When the target peptide is to contain at least one alkyne-containing dicarba bridge and at least one alkene-containing dicarba bridge, it is important to avoid the formation of an intractable mixture of different products from random metathesis between pairs of metathesisable groups or between metathesisable groups and formed dicarba bridges.

It will be appreciated that the step of alkyne metathesis may occur before any number of steps involving alkene metathesis or at the conclusion of any number of steps involving alkene metathesis. However, it is preferred that either the alkyne containing complementary metathesisable groups or the dicarba bridge formed by alkyne metathesis is blocked during any alkene metathesis steps.

As one example, in the formation of a dicarba analogue of insulin having one alkyne-containing dicarba bridge and at least one alkene-containing dicarba bridge, it is possible to introduce a pair of complementary alkyne-containing or alkene-containing metathesisable groups during synthesis of the peptide and to conduct metathesis to form the first dicarba bridge before subsequent pairs of complementary metathesisable groups are introduced into the peptide by peptide synthesis. It may also be necessary to use blocking groups to allow regioselective formation of the first alkyne- or alkene-containing dicarba bridge.

Other examples of typical synthetic routes for the introduction of one alkyne-containing dicarba bridge and one alkene dicarba bridge are shown below. In the catalytic pathways A and B, both combine alkyne and alkene metathesis for the regioselective formation of two dicarba bridges. Both routes involve alkene cross metathesis of a pair of alkene-containing metathesisable groups, alkyne cross metathesis of a pair of alkyne-containing metathesisable groups, and the optional reduction of the newly formed bridges to the corresponding alkanes. The difference between the two pathways is the order of the catalysis: In pathway A, alkyne metathesis (ROAM) precedes alkene (RCM) metathesis, and in pathway B, alkene metathesis precedes alkyne metathesis.

Transition metal metathesis catalysts show varying degrees of chemo-specificity in their activity toward potential substrates. Transition metal alkylidene bearing catalysts, such as those employed in alkene metathesis, show an affinity for coordination and subsequent metathesis of reactable olefins. Some of these catalysts are able to coordinate alkynes and undergo enyne metathesis, the bond reorganisation of an alkyne to form a 1,3-diene (Chem. Rev., 2004, 104(3), pp 1317-1382). These species may react with other alkenes to produce further products. Some alkene metathesis catalysts, such as the first generation Grubbs' catalyst, show limited propensity toward this side reaction in the presence of available alkynes. To avoid unwanted reaction of alkyne moieties the group may be blocked by an appropriate blocking group as described herein (see below).

Transition metal alkylidyne bearing catalysts, such as Schrock's alkyne metathesis catalyst, show high activity toward alkyne metathesis but do not participate in olefin or enyne metathesis (R. R. Schrock, Chem. Commun., 2005, 2773-2777). Hence, protection of olefins during alkyne metathesis is not necessary with catalysts of this kind. Preferably, alkyne metathesis in the presence of alkene-containing metathesisable groups or alkene-containing dicarba bridges is performed with Schrock's catalyst.

Blocking and activation of alkyne-containing metathesisable groups and alkene-containing metathesisable groups are as described above when the dicarba analogue of insulin contains both an alkyne-containing dicarba bridge and an alkene-containing dicarba bridge.

Reduction of Peptides Having Additional Dicarba Bridges

The reduction of alkyne-containing dicarba bridges and alkene-containing dicarba bridges may be performed as described above. It will be appreciated by a person skilled in the art that when multiple dicarba bridges are to be present in a dicarba analogue of insulin, the step of reducing the initially installed alkyne- or alkene-containing dicarba bridge may take place before or after the metathesis reaction to allow reduction of specific dicarba bridges.

Disulfide Bond Formation

The method may also be used to synthesise a peptide or peptides having at least one dicarba bridge as well as one or more disulfide bonds. The disulfide bonds may be introduced at any location, however, it is preferred that they are introduced into a peptide or peptides at locations where disulfide bonds are present in the native peptide.

Where more than one disulfide bridge is to be introduced into the peptide or peptides, disulfide formation, and hence chain combination, must be performed via a regioselective approach. Significantly, undirected ligation of two or more peptide chains is usually only successful in native molecules. When sequence modifications are introduced however, unnatural folding has a deleterious effect on chain combination and resultant topoisomers contaminate and lower the yield of native isomer. One approach is to use orthogonally-protected cysteine residues to sequentially construct the disulfide bridges. An example of the regioselective bond forming strategy was used in the regioselective synthesis of dicarba insulins in Wade et al., J. Biol. Chem., 2006, 281, 34942-34954, which is incorporated herein by reference. Preferably, a combination of complementary thiol protecting groups (e.g. Acm, ^(t)Bu, Trt) are used to regioselectively install disulfide bridges.

Solid Supports

The peptide or peptides used in the preparation of dicarba analogues are preferably attached to a solid support.

A plethora of solid supports are known and available in the art, and include pins, crowns, lanterns and resins. Examples are polystyrene-based resins (sometimes referred to as solid supports), including cross-linked polystyrene resins (via ˜1% divinylbenzene) functionalised with linkers (or handles) to provide reversible linkages between the reactable organic compound (which may be a peptide sequence containing side-chains with cross-metathesisable groups) and the resin. Examples of polystyrene-based resins include Wang resin, Rink amide resin, BHA-Gly-Gly-HMBA resin and 2-chlorotrityl chloride resin. Other forms of solid supports that may not necessarily be characterised as resins can also be used.

Under microwave reaction conditions it is possible to have a higher solid support loading than is conventionally used in peptide synthesis on solid supports. Typical solid support loadings are at the 0.1 mmol/g level, but microwave radiation (optionally combined with solvent choice, as described above) overcomes the aggregation problems at higher solid support loadings, so that solid support loading at around 0.9 mmol/g (nine times higher) is achievable. The solid support loadings may also be at around 0.2 mmol/g and above, such as 0.5 mmol/g and above.

Solvents

The metathesis reaction may be performed in any solvent which provides good catalytic turnover and good resin swell.

Particularly for reactions conducted with the reactable peptide or peptides attached to a solid support such as a resin, metathesis is preferably performed in a solvent combination comprising a resin-swelling solvent with a co-ordinating solvent for the catalyst. In resin-supported reactions, swelling of the resin is required to avoid aggregation and to promote catalyst access to reactive functionality. Some solvents which are suitable for swelling resins are not compatible with metathesis and/or hydrogenation catalysts, and hence careful selection must be made. For example, polystyrene-based resins show optimal swelling in chlorinated solvents such as dichloromethane, however these solvents are not compatible with hydrogenation catalysts. The solvents react with such catalysts to compromise catalyst function—which in turn reduces the catalytic cycle (or turn-over number—TON), resulting in incomplete conversion. It was found that the addition of a small amount of a co-ordinating solvent for the catalyst, such as an alcohol (e.g. methanol, isopropanol) which can co-ordinate into a vacant site of the catalyst to facilitate stability, overcame this problem. The co-ordinating solvent is suitably used in an amount of about 1-30%, for example constituting 10% of the solvent, by volume. The resin swelling agent may be any polar solvent known to swell the resin, such as dichloromethane. Other suitable solvents for a range of resins are as set out in Santini, R., Griffith, M. C. and Qi, M., Tet. Lett., 1998, 39, 8951-8954, the entirety of which is incorporated herein by reference.

Peptide Synthesis

The method for the synthesis of a dicarba bridge in a peptide is described above.

Generally, the peptide will be a protected peptide (such as Fmoc protected), and will comprise a sequence corresponding to a natural or biologically important peptide or a fragment, salt, solvate, derivative, isomer or tautomer thereof. The amino acids used to create the sequence corresponding to a natural or biologically important peptide can be any of the amino acids described earlier, but it is convenient for the synthesis of peptidomimetics for the amino acids to be selected from the 20 naturally-occurring amino acids, γ- and β-amino acids and from any metathesisable group-bearing analogues thereof. An example of metathesisable group-bearing analogues are allylglycine and butynylglycine.

Preferably, at least one of the reactable peptides are provided on a solid support. In this embodiment, the step of adding one or more further amino acids to one or both ends of at least one of the reactable peptides is performed by solid phase peptide synthesis (SPPS).

It will be appreciated that in using the method of the present invention, part of a target peptide or peptides may be synthesised containing one or more metathesisable groups. Catalysis is then performed, which includes metathesis to form the unsaturated dicarba bridge and optionally reduction of the unsaturated dicarba bridge. Once the metathesis has been performed, one or more further amino acids including one or more metathesisable groups may be added to the peptide or peptides. This approach can avoid the need to use blocking groups in some instances. It will also be appreciated that if a peptide sequence is added later through an intermolecular bridge, the corresponding metathesisable groups on that peptide need not be blocked—as they can be added to the reaction at the time of cross-metathesis, after the unblocking of the groups on the solid-supported peptide.

Although the remainder of the description refers to particular examples or embodiments of the invention, it is to be understood that modifications or improvements may be made thereto without departing from the scope of the invention.

Products of Methods

The present application also provides a peptide or peptides produced by the method of the invention. The compound may be a peptide with at least one dicarba bridge, or two or more peptides which together contain and/or are connected by at least one dicarba bridge. These products are also referred to dicarba analogues. Salts, solvates, derivatives, isomers and tautomers of the peptide or peptides are encompassed in this context.

The dicarba analogues may be peptidemimetics of native peptides, where the dicarba bridge has been used to replace a disulfide bond, a salt bridge, an ion pair, a non-covalent interaction (for example, a hydrogen bonding or pi-stacking interaction), or a secondary or tertiary structural feature such as an α-helix, a β-sheet or a non-covalent interaction that is present in the folded peptide.

Preferably, the dicarba bridge is used to replace a disulfide bond forming cysteine amino acid residue pairs. Possible products include dicarba analogues of cystine-containing peptides and dicarba-stabilised α-helix-containing peptides.

Dicarba analogues may be peptides which contain the same amino acid sequence as the native peptide, but with one or more amino acid residue pairs substituted with amino acids bearing a dicarba bridge.

“Native” is a term used to refer to a natural peptide—to be distinguished from the dicarba analogue, or other synthetic analogue, being synthesised. Bis- and higher dicarba analogues are of particular interest, in view of the difficulty in synthesising such compounds. Examples are the dicarba analogues of pharmaceutically important peptides including insulin, octreotide, lanreotide, vapreotide, growth hormone analogues, conotoxins and oxytocin.

EXAMPLES

Various embodiments/aspects of the present invention will now be described with reference to the following non-limiting examples.

Introduction to Alternating SPPS-Catalysis

Cystine bridges are a common method of peptide cyclisation and result from the oxidation of cysteine residues. These bridges are, however, both chemically and metabolically labile and will convert to their constituent amino acids under reducing, nucleophilic and basic conditions. This may lead to a significant change on peptide conformation and loss of biological activity. More recently, unstable cystine links have been successfully replaced with a variety of non-native bridges to yield stable bioactive peptiomimetics. Towards this end, synthetic isosteric units, such as the non-reducible unsaturated and saturated diaminosuberic (Δ⁴Das and Das respectively) and diaminooctynoic acid (Δ⁴Dao and Dao respectively) residues, have become particularly useful in mimicking native disulfide bridges in peptide-based substrates. Conveniently, these dicarba links are readily installed via homogeneous alkene and alkyne catalysed olefin metathesis of allyl- and butynylglycine (Agl and Bgl respectively) residues embedded within a peptide sequence.

The formation of large-membered dicarba rings and/or dicarba peptide analogues of highly hydrophobic sequences, however, is often low yielding due to undesirable physicochemical affects arising from the formation of secondary structures, intermolecular aggregation and an inability to suitably localise both reacting termini. There are several known strategies for addressing this problem. For example, the introduction of turn-inducing residues into the primary sequence of a peptide is known to disrupt these unfavourable secondary structures and aggregation. The synthesis-derived oxazolidines and thiazolidines, so called pseudoproline residues (ψPro), serve as structural mimetics of native proline. ψPro residues act as masked equivalents of threonine/serine and cysteine respectively and adopt desirable cis-configured amide bonds to promote ring closing metathesis (RCM). Using these cyclic building blocks, near quantitative N→C yields (i.e. head to tail cyclisation) can be achieved in peptides that are virtually unobtainable using conventional cyclisation conditions. Others have also applied the ψPro backbone protection strategy to solid-phase RCM reactions and achieved dicarba peptides in high yield. Other strategies for enhancing dicarba bridge formation via metathesis includes the use of microwave irradiation (pwave) and/or the addition of chaotropic salts to catalysis reactions.

In a growing number of cases, however, we have found that implementation of all of the above strategies can still fail to produce high cross metathesis yields. We have found that the success of a given metathesis-driven cyclisation or cross reaction is highly dependent on the nature of the linear peptide sequence. The synthesis of linear human insulin A chain, for example, is a difficult sequence to prepare and crude sequence mixtures are often contaminated with deletion products (truncated peptides) arising from incomplete amino acid couplings due to deleterious aggregation. Furthermore, in this insulin example, the final five N-terminal residues, GIVEQ, introduce a highly hydrophobic section to the peptide sequence. We reasoned at if we could conduct alternating SPPS-Catalysis-SPPS on a resin-bound truncated sequence (i.e. without the complication of the N-terminal section), we would be successful in achieving high cyclisation yield and be able to readily introduce the strategically omitted residues following catalysis by standard SPPS. We have found that Alternating SPPS-Catalysis-SPPS is highly effective for the production of dicarba peptidomimetics (see FIGS. 1-5).

Solid Phase Peptide Synthesis and Catalysis

The native insulin A-chain is considered to be a “difficult sequence” and is used to illustrate the new methodology. Strongly hydrophobic sequences, such as those found in transmembrane peptides, are prone to significant β-sheet formation and aggregation which leads to poor coupling efficiencies and solvation. Although the A-chain of insulin consists of mainly polar residues, their side chains are reactive and must be protected with hydrophobic groups such as ^(t)Bu and Trt during synthesis. In this sense, the A-chain is also considered highly hydrophobic and is therefore vulnerable to deleterious secondary structure formation. The use of microwave-accelerated SPPS has facilitated the production of linear insulin A-chain in high yield and purity.

Construction of the linear [6,11]-Agl-[7]-Cys(^(t)Bu)-[20]-Cys(Acm) analogue 1 of the human insulin A-chain was performed on preloaded Fmoc-Asn(Trt)-PEG-PS resin. This hybrid resin consists of a low resin substitution and hydrophilic polyethylene glycol linkers which help to decrease deleterious crowding of the elongating peptide chains near the polystyrene solid support. It was thought that a change in these two parameters may reduce some of the undesirable hydrogen-bonded secondary structures about the resin, resulting in higher yielding sequence synthesis and possibly greater catalysis yields (e.g. in CM/RCM/RCAM/H reactions).

Microwave-accelerated SPPS using HATU-DIPEA activation and Fmoc-protected amino acids were used to construct the desired sequence, carrying through each intermediate without purification and characterisation. Two strategically placed Agl residues were incorporated into the primary sequence to facilitate the formation of insulin intra-chain dicarba bridge. In addition, cysteine residues with orthogonal protection were incorporated to allow regioselective disulfide oxidation between the A- and B-chains of insulin. After complete construction of the A-chain, a small aliquot of the resin-tethered peptide was subjected to TFA-mediated cleavage and mass spectral analysis of the resultant solid gave molecular ion peaks at m/z 1249.5 [M+2H]²⁺ and 833.5 [M+3H]³⁺, consistent with the formation of peptide 1.

Ring closing metathesis of the fully protected, resin-tethered linear peptide 1 was performed in the presence of 20 mol % 2^(nd) generation Grubbs' catalyst in DCM. Lithium chloride (as a 0.4 M solution in DMF) was added to the reaction mixture to assist in the reduction of peptide aggregation and aid reagent penetration. However, microwave irradiation of the peptide-resin at 100° C. for 2 h resulted in only 40% conversion (by RP-HPLC) to the desired cyclic peptide 2 (2(I) and 2(II) (see Table below). Mass spectral analysis of the TFA-cleaved product mixture gave the required molecular ions with m/z 1235.7 [M+2H]²⁺ and 824.5 [M+3H]³⁺. The RP-HPLC trace showed the formation of two geometric isomers with two distinct retention times. Fmoc-deprotection and full TFA-cleavage of the remaining resin-bound peptide 2 provided the target cyclic human insulin (hINS) A-chain (Scheme 1).

Several strategies were next examined to increase the metathesis conversion. The resin-bound peptide sequence was exposed to several sequential catalysis cycles but showed no improvement in conversion. Disappointingly, changes to reaction solvent, temperature and time also had no positive effect on cyclisation conversion (see Table below). The use of different Ru-alkylidene catalysts also had no significant effect on RCM yield. Conventional and microwave induced heating was examined but produced no solution to the low conversion problem. A summary of trialled reactions is tabulated below:—

TABLE Optimisation of the ring closing metathesis yield of linear peptide 1 and 3 1

3

Temper- Con- RCM ature Duration version Substrate Catalyst Heat Source Solvent (° C.) (h) (%) 3 20 mol % Conventional DCM  38 65 20 to 5 2^(nd) gen. Grubbs' catalyst 3 20 mol % Conventional DCE  83 65 25 to 5 2^(nd) gen. Grubbs' catalyst 3 20 mol % Microwave DCM 100  1 50 to 5 2^(nd) gen. Grubbs' catalyst 1 20 mol % Microwave DCM 100  1 25 to 2 2^(nd) gen. Grubbs' catalyst 1 20 mol % Microwave DCM 100  2 30 to 2 2^(nd) gen. Grubbs' catalyst 1 20 mol % Microwave DCM 100  3 30 to 2 2^(nd) gen. Grubbs' catalyst 1 20 mol % Microwave DCM 100  2 40 to 2 2^(nd) gen. Hoveyda- Grubbs' catalyst Note: All reaction attempts included 5% 0.4 M LiCl in DMF as a solvent additive.

Despite the relatively low metathesis conversion, homogeneous hydrogenation of the mixture containing the newly formed unsaturated carbocycle was attempted. Rh(I)-phosphine catalysed hydrogenation, using Wilkinson's catalyst, successfully reduced the resin-bound linear peptide 1 containing the two unreacted allylglycine residues. This reduction indicated that the catalyst was active. The accompanying carbocycle 2, however, under conventional heating conditions, failed to undergo hydrogenation to 4 (under varying hydrogen pressure, reaction temperature and duration) (see Table below). Repeated exposure of the resin-bound sequence to fresh catalyst did not improve the conversion, nor did changes to reaction temperature or time.

TABLE Optimisation of the hydrogenation yield in carbocyclic peptides 2 and 5 2

5

H₂ Heat Temperature Duration Pressure Conversion substrate Source Solvent (° C.) (h) (psi) (%) 2 Conventional DCM:MeOH r.t. 68 90    0 to 4 (9:1) 2 Microwave DCM:MeOH 50  2 60 <20 to 4 (9:1) 5 Conventional DCM:MeOH r.t. 18 60 0 (9:1) 5 Conventional DCM:MeOH r.t. 72 90  0* (9:1) 5 Conventional DCM:MeOH r.t. 18 90 0 (9:1) 5 Conventional DCM:MeOH 38 18 90 0 (9:1) 5 Microwave DCM:MeOH 50  2 60   20 to 6 (9:1) 5 Microwave DCM:MeOH 50  4 60   50 to 6 (9:1) Note: All reaction attempts included 5% LiCl (0.4 M) in DMF as a solvent additive; *Catalysis performed in solution on a cleaved peptide.

Hydrogenation of the unsaturated carbocycle 5 was also attempted. Model studies had previously shown that the prenyl substituent was inert to hydrogenation conditions which were normally suitable for reduction of the disubstituted C═C bridge. Under conventional heating conditions, the reaction failed; under microwave irradiation, a modest yield (typically less than 50%) of the reduced carbocycle 6 could be obtained. In this case the prenyl group remained intact (Scheme 3).

Despite the incomplete hydrogenation of 5, activation of the prenyl substituent of 6 via cross metathesis with cis 2-butene was attempted. In all attempts to yield 7, 0% conversion of the prenyl→crotyl sidechain was obtained (Scheme 3).

Alternating Solid Phase Peptide Synthesis and Catalysis

The above described catalysis on resin-bound peptides highlights the catalysis challenges that can be experienced with ‘difficult sequences’. While insulin A chain is used as an illustrative example, some peptide sequences will only undergo metathesis reactions in low yield. While experimental conditions (as described above) can be modified to enhance conversion, incomplete conversion poses significant downstream problems particularly in regard to maintaining regioselectivity and purification of the peptide target.

To rectify this growing problem we considered an alternating SPPS-Catalysis-SPPS approach where truncated peptide sequences (i.e. incomplete sequences containing one or more metathesisable residues) were constructed on resin via standard solid phase peptide synthesis (SPPS). Following catalysis of the resin-bound peptide (e.g. RCM, ROAM, CM, H), the peptide would then be chain extended via a second round of SPPS. Significantly, this approach is highly generic and enables the synthesis and catalysis (e.g. metathesis or hydrogenation (H)) of ‘difficult sequences.’

To illustrate this approach, the above insulin A-chain sequence was reconstructed on resin and five of the N-terminal residues (i.e. GIVEQ) were omitted from the sequence. Hence, two linear peptides 8 and 9 (Note: Peptide 8 is a desA₁₋₅ analogue of 1) were constructed without the GIVEQ residues. Under analogous experimental conditions to those described above, these truncated sequences underwent near quantitative catalysis (RCM, CM, H: as described below) (Schemes 4, 5 and 6) and the remaining five residues were then appended to the N-terminus following catalysis (Schemes 6 and 7) via continued SPPS synthesis to give peptides 4, 16 (and also 2, 26 and 43—see Experimental section). Significantly, it is important to note that the full chain analogue of 8 (i.e. 1) only underwent RCM to a maximum of 40% and completely resisted hydrogenation to yield peptide 4. This is in contrast to the metathesis and hydrogenation of the truncated sequence 8 (i.e. 1—GIVEQ) which gave a >95% and 91% conversion respectively. This result highlights the benefit of the alternating SPPS-Catalysis-SPPS approach.

TABLE Ring closing metathesis Temper- Con- RCM Heat ature Duration version Substrate Catalyst Source Solvent (° C.) (h) (%) 8 20 mol % Microwave DCM 100 2 >95% to 2^(nd) gen. 10 Grubbs' catalyst 9 20 mol % Microwave DCM 100 2 >95% to 2^(nd) gen. 11 Grubbs' catalyst

Example

TABLE Hydrogenation Temper- Pres- Con- H₂ Heat ature Duration sure version Substrate Source Solvent (° C.) (h) (psi) (%) 10 Microwave DCM:MeOH 70 2 90  91 to 12 (9:1) 11 Microwave DCM:MeOH 70 2 80 100 to 13 (9:1)

Example

Prenyl Activation: A Second Metathesis Reaction

Following reduction of the metathesis installed dicarba bridge in 11, formed from resin-bound precursor 9, the prenyl group can be activated in 13 for further dicarba bridge formation via a cross metathesis reaction with cis-2-butene. Notably, this reaction failed on the complete insulin A-chain (described above). With the five N-terminal residues deleted, however, the prenyl group was found to be accessible for the first time and underwent the required cross metathesis reaction to give resin-tethered peptide 14 with excellent conversion (Scheme 6). This is an example of an intermolecular cross metathesis reaction. Once activated, the generated crotylglycine substituent was reacted with N-acetyl allylglycine methyl ester to form a second dicarba bridge (i.e. 15) analogous to that found between the A and B chains of insulin.

Once again, addition of the final five residues (i.e. GIVEQ) to the N-terminus of 15 following the multiple catalysis transformations was straightforward (Scheme 6). This final transformation yielded the bis-dicarba peptide 16. This example (i.e. from 9→11→13→14→15→16) convincingly shows the benefit of the alternating SPPS-Catalysis-SPPS approach.

Continued SPPS

As per the above example, once the required catalysis has been conducted on the resin-tethered sequences, the N-terminal protecting group is readily removed to facilitate SPPS construction of the full sequence (e.g. the addition of the remaining five N-terminal residues, GIVEQ, in the case above). Hence, additional residues were readily attached to the N-terminus of other carbocyclic peptides via this approach (Scheme 6 and 7).

Hence, alternating SPPS-catalysis-SPPS can be used to generate dicarba peptide analogues of sequences that are reluctant to undergo ring closure (and hydrogenation) via existing methodologies. In addition, alternating SPPS-catalysis-SPPS can be used to generate dicarba analogues of sequences that are reluctant to undergo cross metathesis (and subsequent hydrogenation) via existing methodologies. As one example, for compound 15, a residue may be attached via a dicarba bridge to the A-chain to facilitate growth of a second chain.

Rapid and long acting synthetic insulins are available for the treatment of diabetes. These analogues, such as Lispro and Glargine, are modeled on the two-chain structure of human insulin and possess residue changes in the 30-residue B chain to alter physicochemical absorption properties. The C-terminal A-chain arginine residue is also replaced with glycine to enhance formulation stability of the insulin at low pH. The alternating SPPS-catalysis-SPPS method can also be employed to generate dicarba analogues of these synthetic insulins. The method is particularly important for the generation of A6-A11 unsaturated and saturated analogues as RCM and hydrogenation on full A-chain sequences is generally not viable. Several unsaturated and saturated dicarba-insulin analogues have been prepared using the SPPS-catalysis-SPPS approach and detailed in full in the Experimental Section.

Alternating Catalysis SPPS Using Alkyne Metathesis In addition, the alternating SPPS-Catalysis-SPPS method can also be applied to resin-bound peptide sequences prepared for ring closing alkyne metathesis. Sequence inclusion of two butynylglycine residues can be used to form an intramolecular alkyne bridge via alkyne metathesis. Solid phase peptide synthesis was used to construct octapeptide 17 (X=Thr). Exposure of the resin-bound peptide to Schrock's catalyst (microwave, 35 W, 70° C., 3 h) yielded the target cyclic peptide 18 in 45% yield. Significantly, substitution of the threonine residue with a turn-inducing proline residue (X=Pro, 19) gave a peptide which underwent excellent ring closing alkyne metathesis (ROAM) (80%) under analogous conditions to yield cyclic peptide 20 (Scheme 8). This example illustrates the importance of suitably localizing reactive termini for high ring closing yield. Pseudoproline residues, as used in alkene metathesis, are also invaluable for alkyne metathesis. Significantly, this reaction fails when the N-terminal GIVEQ residues are attached prior to the catalysis. On the other hand, when these residues are initially deleted from the sequence exposed to the catalyst, the ROAM (and optional hydrogenation) proceeds with high conversion. The initially deleted N-terminal residues are then added to the sequence following catalysis. Hence, addition of the remaining five residues yields peptide 21 in a straightforward manner (Scheme 8).

Removable Tethers: An RCM Approach

In some instances and for some sequences it may be difficult to form an intermolecular dicarba bridge. This approach has been tested for preparation of interchain dicarba insulin analogues.

Native insulins are generated from a single-chain proinsulin precursor (A-C-B), where the inserted C-peptide sequence, varying in length from 26-38 residues, aids native disulfide folding (Scheme 9, left). Taking a lead from nature, a removable, turn-inducing truncated sequence or residue (X) could be inserted between the A- and B-chains to promote formation of the target A7-B7 interchain dicarba bridge via RCM (Scheme 9, right). Following bridge formation, the X residue/s would then be removed, and ligation of the remaining B9-B21 segment via convergent SPPS would provide the complete 51 amino acid dicarba insulin target.

This idea was initially investigated using a truncated A-chain sequence, and a readily available turn-inducing proline residue as the tether (X). The 17 residue sequence B1-8+Pro (═X)+A1-8 140 was constructed using microwave-accelerated SPPS onto Fmoc-Thr(^(t)Bu)-PEG-PS resin. The B-chain was truncated at the Gly_(B8) residue to minimise the size of the resulting cycle, and to provide a racemisation free C-terminal residue for future convergent SPPS. Microwave-accelerated RCM of the resin-tethered peptide in the presence of 20 mol % second generation Grubbs' catalyst gave 78% conversion to the target carbocycle 141 (Scheme 10).

Notably, when RCM was performed on the peptide with its six N-terminal amino acids omitted, 142, RCM proceeded with quantitative conversion. Following catalysis, the remaining residues were readily appended to the N-terminus via microwave-accelerated SPPS to give target peptide 141 (Scheme 11).

We also sought to replace the stable proline linker (X) with a turn-promoting, removable tether to generate the target A7-B7 dicarba link and native N-terminal Gly_(A1) residue. A hydroxy-6-nitrobenzaldehyde (HnB) residue 143, was selected for the role. This aldehyde is readily incorporated at the N-terminus of a peptide sequence via reductive amination, and promotes head-to-tail cyclisation reactions through the introduction of cisodial geometry in the peptide backbone. Furthermore, the auxiliary is reported to undergo efficient and selective removal by mild photolysis.

The HnB residue 143 could be used in a novel way, via introduction between the allylglycine-containing insulin A- and B-chains to promote RCM. Following cross metathesis and additional sequence construction, cleavage of the HnB auxiliary via sequential hydrolysis and photolysis would then yield the A-B heterodimer with the target A7-B7 dicarba bridge (Scheme 12).

The photolabile HnB tether 143 was synthesised from meta-nitrophenol 144 in a revised three step reaction sequence (Scheme 13). This aldehyde was then successfully introduced into the truncated A-chain sequence (A1-A8) 145, via reductive amination, to give linear peptide 146 in 90% yield. The resultant secondary amine was then protected prior to esterification with an activated Fmoc-Gly-OH residue. Mass spectral analysis of the TFA-cleaved product mixture following estification showed the required sequence 147 in addition to starting peptide 146. Coupling of the second allylglycine residue and subsequent RCM with 20 mol % second generation Grubbs' catalyst was successful. The mass spectrum of a cleaved aliquot of resin showed, inter alia, the target ring closed product 148 with m/z 1431.5.

The interrupted SPPS-catalysis approach is also exemplified with peptide 68, a 19-amino acid sequence composed of mainly hydrophobic residues and three 17-membered rings. High yielding construction of this peptide from a full linear sequence (Agl-API-Agl-SL-Agl-API-Agl-SL-Agl-API-Agl-SLG) would not be successful. Furthermore, even RCM on a truncated 11-mer (ASL-Agl-API-Agl-SLG), where the N-terminal allylglycine residue is flanked by three adjacent N-terminal residues, is problematic. Only poor conversion (28%) was observed under optimum metathesis conditions and reduction of the newly installed C═C linkage was also incomplete. Even in a single ring formation a retarded metathesis reaction complicates product purification (i.e. each metathesis cycle generates a mixture of two isomeric C═C peptides (E- and Z) and the starting linear material); this problem then compounds with each additional ring. The inability to hydrogenate the product also complicates purification (i.e. 8 isomers would comprise a ‘pure’ tricyclic peptide mixture) and reduces yield because both isomers can not be reduced to the same saturated carbocycle. Furthermore, the inability to reduce the newly installed C═C can also complicate subsequent RCM reactions and result in ring-opening metathesis and hence loss of topoisomer selectivity. Significantly, we have found that temporary exclusion of the N-terminal residues not involved in ring formation, i.e. terminating the sequence at the second allylglycine residue, can greatly assist both Ru-alkylidene-catalysed RCM and hydrogenation. Without the ‘tail’ residues, the peptide is able to adopt favourable conformation for RCM and catalyst penetration. The newly formed C═C bridge is also readily reduced under homogeneous Rh(I)-catalysed hydrogenation conditions when the N-terminal residues are absent. Since both these steps are conducted on the solid-phase, the peptide can be readily subjected to further SPPS after catalysis to incorporate residues for a second carbocycle. The above described catalysis steps are then repeated to incorporate a third carbocycle. Importantly, the peptide remains attached to the resin throughout the catalytic cycles and is immediately available for additional rounds of SPPS. Final cleavage from the resin and preparative RP-HPLC purification yielded the target example tricyclic peptide 68 as described below:

The interrupted SPPS-catalysis approach has also been successfully employed to perform successive CM reactions on resin-tethered sequences. This idea is exemplified with the synthesis of peptide 85 shown below. The intermediate peptide 83 can also be extended to incorporate two metathesisable groups for construction of a carbocyclic ring. This example illustrates that the interrupted SPPS-catalysis approach can be used to conduct sequential and regioselective CM and RCM within a single peptide sequence to unambiguously install two dicarba bridges.

In an analogous fashion, the interrupted SPPS-catalysis approach can be used to install residues for construction of a secondary peptide chain. The following example illustrates how regioselective CM and RCM can be conducted within a single peptide to generate intra- and inter-molecular dicarba bridges which closely mimic the native cysteine (disulfide) bridge. This is important for the synthesis of dicarba analogues of bioactive molecules such as insulin.

Alternating Peptide-Catalysis on truncated peptide sequences is a highly effective and generic method for the synthesis of dicarba peptides. This new approach leads to high metathesis yield by maximizing catalyst accessibility to reactive motifs and eliminates low yielding RCM/RCAM, CM/CAM and subsequent hydrogenation due to deleterious sequence aggregation. The approach facilitates the introduction of strategically omitted residues following catalysis by standard peptide synthesis. This method can be applied to peptides supported on a solid phase to rapidly provide single and multi-dicarba peptides.

General Experimental

Instrumentation

Low resolution electrospray ionisation (ESI) mass spectra were recorded on a Micromass Platform Electrospray mass spectrometer (QMS-quadrupole mass electroscopy) as solutions in specified solvents. Spectra were recorded in the positive and/or negative mode (ESI⁺/ESI⁻). The mass spectrometers were calibrated with an internal standard solution of sodium iodide in MeOH.

Reverse phase high performance liquid chromatography (RP-HPLC) was performed on Agilent 1200 instruments. For analytical runs, instruments were equipped with photodiode array (PDA) detection (controlled by ChemStation software) and an automated injector (100 μL loop volume). In preparative runs, instruments used multivariable wavelength (MVW) detection (controlled by ChemStation software) and an Agilent unit injector (2 mL loop volume). The solvent system used throughout this study (except where specified) was buffer A: 0.1% aqueous TFA; buffer B: 0.1% TFA in MeCN. Analytical experiments were carried out on Vydac C4 or C18 (4.6×250 mm, 5 μm) analytical columns, at a flow rate of 1.5 mL min⁻¹. Preparative RP-HPLC was carried out on Vydac C4 or C18 (22×250 mm, 10 μm) preparative columns, at a flow rate of 10 mL min⁻¹. Linear gradients of 0.1% TFA in MeCN were employed as specified.

Microwave ring closing metathesis (e.g. RCM/RCAM), cross metathesis (CM) and hydrogenation reactions were carried out on a CEM Discover™ system fitted with the Benchmate™ or Gas Addition™ option. The instrument produces a continuous focussed beam of microwave radiation at a maximum power delivery selected by the user, which reaches and maintains a selected temperature. Reactions were performed in 10 mL high pressure glass microwave vessels fitted with self-sealing Teflon septa as a pressure relief device. The vessels employ magnetic stirrer beads and the temperature of each reaction was monitored continuously with a non-contact infrared sensor located below the microwave cavity floor, or a using a glass enclosed fibre optic temperature probe. Reaction times were measured from the time the microwave reached its maximum temperature until the reaction period had lapsed (cooling periods not inclusive).

Solvents and Reagents

Acetone, acetonitrile (MeCN), ammonium bicarbonate (NH₄HCO₃), concentrated hydrochloric acid (conc. HCl), 1,2-dichloroethane (DCE), dichloromethane (DCM), dimethyl sulfoxide (DMSO), ethanol (EtOH), ethyl acetate (EtOAc), diethyl ether (Et₂O), glacial acetic acid (AcOH), hexanes (light petroleum), iso-propanol (i-PrOH), lithium chloride (LiCl), methanol (MeOH), pyridine, tetrahydrofuran (THF), triethylamine (Et₃N), toluene, magnesium sulfate (MgSO₄), sodium chloride (NaCl), sodium bicarbonate (NaHCO₃), sodium carbonate (Na₂CO₃) and sodium hydroxide (NaOH) were used as supplied by Merck. Anhydrous THF and Et₂O were stored over sodium (Na) wire and distilled from Na and benzophenone prior to use. DCM was dried over calcium chloride (CaCl₂) and distilled from calcium hydride (CaH) prior to use. Toluene was stored over Na wire and distilled prior to use. Common reagents were used as supplied by Aldrich. Deuterated solvents were used as supplied by Cambridge Isotopes Laboratories.

Solid Phase Peptide Synthesis Procedures

Peptide Materials and Reagents

Automated microwave-accelerated solid-phase peptide synthesis (SPPS) was carried out using a CEM Liberty-Discover™ system. Manual SPPS was performed in polypropylene Terumo syringes (5 or 10 mL) fitted with a polyethylene porous (20 μm) filter. Resin wash and filtering steps were aided by the use of a Visprep™ SPE DL 24-port model vacuum manifold as supplied by Supelco. Coupling reactions and cleavage mixtures were shaken on a KS125 basic KA elliptical shaker supplied by Labortechnik at 400 revolutions per minute (rpm). Cleaved peptides were collected by centrifugation at a speed of 6,000 rpm, on a Hermle Z200A centrifuge supplied by Medos or at a speed of 6,000 rpm, on a TMC-1 mini centrifuge supplied by Thermoline.

4-Amino-2-pentenoic acid (allylglycine, Agl) was used as supplied by Peptech. Trifluoroacetic acid (TFA) and N,N-dimethylformamide (DMF) were supplied by Auspep and the latter was stored over 4 Å molecular sieves. Dichloromethane (DCM) and piperidine were supplied by Merck and stored over 4 Å molecular sieves. N,N-Diisopropylcarbodiimide (DIC), diisopropylethylamine (DIPEA), 4-dimethylaminopyridine (DMAP), 2,2′-dipyridyl disulfide (2,2′-DPDS), N-methylmorpholine (NMM), N-methyl-2-pyrrolidone (NMP), thioanisole, trifluoromethanesulfonic acid (TfOH) and triisopropylsilane (TIPS) were used as supplied by Aldrich. N-Fluorenylmethoxycarbonylaminosuccinimide (Fmoc-OSu), O-(7-azabenzotriazol-1-yl)-N,N,N′,N′-tetramethyluronium hexafluorophosphate (HATU), O-(benzotriazol-1-yl)-N,N,N′,N′-tetramethyluronium hexafluorophosphate (HBTU), N-hydroxybenzotriazol (HOBt) and Wang, Fmoc-Phe-Wang, Fmoc-Arg(Pbf)-Wang, Fmoc-Gly-Wang, rink amide, H-Thr(^(t)Bu)-2-chlorotrityl chloride and 2-chlorotrityl chloride resins were used as supplied by GL Biochem. Fmoc-amino acids were also used as supplied by GL Biochem and reactive sidechains were protected with the Acm, Boc, Pbf, ^(t)Bu, and Tit protection groups. Fmoc-Asn(Trt)-PEG-PS and Fmoc-Thr(^(t)Bu)-PEG-PS resin was used as supplied by Applied Biosystems.

Manual Peptide Synthesis

Manual SPPS was carried out using fritted plastic syringes, allowing filtration of solution without the loss of resin. The tap fitted syringes were attached to a vacuum tank and all washings were removed in vacuo. This involved soaking the resin in the required solvent for a reported period of time followed by evacuation to allow the removal of excess reagents before subsequent coupling reactions.

Esterification of Wang Resin

In a fritted syringe, Wang resin was swollen with DCM (4 mL; 3×1 min, 1×60 min) and DMF (4 mL; 3×1 min, 1×30 min). Prior to coupling, the C-terminal amino acid was activated by addition of DIC (3 equiv.) to a solution of the protected amino acid residue, Fmoc-L-Xaa-OH (3 equiv.), in DMF (3 mL). This activated amino acid solution was added to the swollen resin and shaken gently for 1 min. A solution of DMAP (0.3 equiv.) in DMF (1 mL) was then added to the resin, and the reaction mixture was shaken gently for a further 2-18 h. At the end of this reaction period, the mixture was filtered and the resin-tethered amino acid was washed with DMF (4 mL; 3×1 min). To prevent the formation of truncated sequences, any remaining active sites were capped with an acetic anhydride capping solution (4 mL; DMF:acetic anhydride:NMM; 94:5:1) before being filtered and washed with DMF (4 mL; 3×1 min). Additional amino acid coupling were then carried out via either manual or automated microwave-accelerated SPPS.

Automated Microwave Peptide Synthesis

Automated microwave-accelerated SPPS was carried out using a CEM Liberty-Discover™ synthesiser. This involved the flow of dissolved reagents from external nitrogen pressurised bottles to a resin-containing microwave reactor vessel fitted with a porous filter. Coupling and deprotection reactions were carried out within this vessel and were aided by microwave energy. Each reagent delivery, wash and evacuation step was carried out according to automated protocols of the instrument controlled by PepDriver software.

In a 50 mL centrifuge tube, the resin was swollen with DMF:DCM (10 mL; 1:1; 1×60 min) and connected to the Liberty™ resin manifold. The Fmoc-amino acids (0.2 M in DMF), activators (0.5 M HBTU/HOBt or HATU in DMF), activator base (2 M DIPEA in NMP) and deprotection agent (20% v/v piperidine in DMF) were measured out and solubilised in an appropriate volume of specified solvent as calculated by the PepDriver software program. The default microwave conditions used in the synthesis of each linear peptide included: Initial deprotection (36 W, 37° C., 2 min), deprotection (45 W, 75° C., 10 min), preactivation (0 W, 25° C., 2 min) and coupling (25 W, 75° C., 10 min). Each arginine residue underwent a double coupling involving the filtration and delivery of fresh reagents and a second preactivation (0 W, 25° C., 2 min) and coupling (25 W, 75° C., 10 min) step. Cystine and histidine residues were subjected to modified and lower temperature microwave conditions including: Initial deprotection (50 W, 37° C., 120 min), deprotection (50 W, 75° C., 600 min), preactivation (0 W, 25° C., 120 min) and coupling (25 W, 50° C., 600 min). On synthesis completion, the resin-bound peptides were automatically returned to the Liberty™ resin manifold as a suspension in DMF:DCM (1:1) and filtered through fritted plastic syringes (10 mL) prior to acid-mediated cleavage (General Section).

Resin Cleavage Procedure

A small aliquot of resin-bound peptide (approx. 5 mg) was suspended in a cleavage solution (1 mL; TFA:TIPS:water:thioanisol; 95:2:2:1) and shaken gently for 2 h. The mixture was filtered through a fritted syringe and the beads rinsed with TFA (1×0.2 mL). The filtrate was concentrated under a constant stream of air and the resultant oil was induced to precipitate in ice-cold Et₂O (1 mL). Cleaved peptides were collected by centrifugation (3×5 min) and dried for analysis by analytical RP-HPLC and mass spectrometry. For full scale resin cleavages, 10 mL of cleavage solution was used and after 4 h, the resin was rinsed with TFA (3×2 mL). The filtrate was concentrated under a constant stream of air and the resultant oil was induced to precipitate in ice-cold Et₂O (35 mL). Collection by centrifugation was carried out over 5×6 min spin times.

Metathesis Procedures

Catalysts and Materials

Catalysts:

Tricyclohexylphosphine[1,3-bis(2,4,6-trimethylphenyl)-4,5-dihydro-imidazol-2-ylidene](benzylidene)ruthenium(II) dichloride (2^(nd) generation Grubbs' catalyst) and (1,3-Bis-(2,4,6-trimethylphenyl)-2-imidazolidinylidene)dichloro(o-isopropoxyphenylmethylene)ruthenium(II) (2^(nd) generation Hoveyda-Grubbs' catalyst) were used as supplied by Aldrich. Tris(tert-butoxy)(2,2-dimethylpropylidyne)tungsten(VI) (Schrock's catalyst) was used as supplied by Strem Chemicals Inc. and stored under nitrogen in a sealed ampoule at −10° C. (protected from light).

Volatile Olefins:

2-Methyl-2-butene and cis-2-butene were used as supplied by Aldrich.

Solvents:

DCM, DCE, DCB and a 0.4 M solution of LiCl in DMF used in all metal-catalysed metathesis reactions were degassed with high purity argon in addition to the general freeze-pump-thaw procedure (General Section) prior to use.

Reaction Vessels:

Schlenk tubes or CEM Benchmate™ microwave reactor vessels were employed for ring closing and cross metathesis reactions involving the use of solid or liquid (non-volatile) reactants. Shield aerosol pressure reactors (Fischer-Porter tubes) (100 mL) fitted with pressure gauge heads or CEM Gas Addition™ microwave reactor vessels were used for CM reactions involving gaseous (cis-2-butene and) or volatile (2-methyl-2-butene) reactants. All vessels were equipped with stirrer beads.

Conventional Ring Closing and Cross Metathesis Procedures (Non-Volatile Reactant)

A Schlenk tube was loaded with substrate, deoxygenated solvent, deoxygenated solvent additive and catalyst in an inert (argon or nitrogen) environment. The reaction mixture was stirred at reflux (Δ) for a specified period of time and then cooled to room temperature. In solution-phase experiments, volatile species were removed under reduced pressure and the crude product was purified by column chromatography, where necessary, to give the required product. Metathesis reactions on the solid-phase were filtered through a fritted syringe, washed with DCM (3-7 mL; 3×1 min) and MeOH (3-7 mL; 3×1 min), then dried in vacuo for 1 h. The resin was subjected to acid-mediated cleavage (General Section) and the resultant isolated solid was then analysed by RP-HPLC and mass spectrometry.

Conventional Cross Metathesis Procedure (Volatile Reactant)

A Fischer-Porter tube was loaded with substrate, deoxygenated solvent, deoxygenated solvent additive, catalyst and reacting volatile olefin in an inert (nitrogen or argon) environment. The system was sealed and the reaction mixture then stirred at reflux (A) for a specified period of time and then cooled to room temperature. In solution-phase experiments, volatile species were removed under reduced pressure and the crude product was purified by column chromatography, where necessary, to give the required product. Metathesis reactions on the solid-phase were filtered through a fritted syringe, washed with DCM (3-7 mL; 3×1 min) and MeOH (3-7 mL; 3×1 min), then dried in vacuo for 1 h. The resin was subjected to acid-mediated cleavage (General Section) and the resultant isolated solid was then analysed by RP-HPLC and mass spectrometry.

Conventional Cross Metathesis Procedure (Gaseous Reactant)

A Fischer-Porter tube was loaded with substrate, deoxygenated solvent, deoxygenated solvent additive and catalyst under an inert (argon or nitrogen) environment. The system was sealed and the pressure vessel then connected to a vacuum manifold and purged three times using vacuum and argon flushing cycles. After charging the vessel with a gaseous olefinic reactant to the reported pressure, the reaction mixture was stirred at reflux (Δ) for a specified period of time. Once cooled to room temperature, solution-based reaction mixtures were concentrated under reduced pressure and purified by column chromatography, where necessary, to give the required product. Resin-based experiments were filtered through a fritted syringe, washed with DCM (3-7 mL; 3×1 min) and MeOH (3-7 mL; 3×1 min), then dried in vacuo for 1 h. The resin was subjected to acid-mediated cleavage (General Section) and the resultant isolated solid was then analysed by RP-HPLC and mass spectrometry.

Microwave-Accelerated Ring Closing and Cross Metathesis Procedure

A microwave reactor vessel was loaded with substrate, deoxygenated solvent, deoxygenated solvent additive, catalyst and reacting olefin in an inert (nitrogen or argon) environment. The system was sealed and the reaction mixture then irradiated with microwave energy whilst being stirred at specified temperature for a specified period of time. After cooling to room temperature, solution-based reaction mixtures were concentrated under reduced pressure and purified by column chromatography, where necessary, to give the required product. Resin-based experiments were filtered through a fritted syringe, washed with DCM (3-7 mL; 3×1 min) and MeOH (3-7 mL; 3×1 min), then dried in vacuo for 1 h. The resin was subjected to acid-mediated cleavage (General Section) and the resultant isolated solid was then analysed by RP-HPLC and mass spectrometry.

Microwave-Accelerated Cross Metathesis Procedure (Gaseous Reactant)

Microwave-accelerated cross metathesis involving volatile reactants was carried out on a CEM Discover™ system fitted with the Gas Addition™ option as described in the General Section. A microwave reactor vessel was loaded with substrate, deoxygenated solvent, deoxygenated solvent additive and catalyst in an inert (argon or nitrogen) environment. The system was sealed and the pressure vessel then connected to a vacuum manifold and purged three times using vacuum and argon flushing cycles. After charging the vessel with a gaseous olefinic reactant to the reported pressure, the reaction mixture was irradiated with microwave energy whilst being stirred at the specified temperature for a specified period of time. After cooling the reaction mixture to room temperature, the gas was vented from the system. In solution-phase experiments, volatile species were removed under reduced pressure and the crude product was purified by column chromatography, where necessary, to give the required product. Metathesis reactions on the solid-phase were filtered through a fritted syringe, washed with DCM (3-7 mL; 3×1 min) and MeOH (3-7 mL; 3×1 min), then dried in vacuo for 1 h. The resin was subjected to acid-mediated cleavage (General Section) and the resultant isolated solid was then analysed by RP-HPLC and mass spectrometry.

Metathesis experiments are described using the following format: substrate (mg), solvent (mL), additive, catalyst (mg), reacting olefin (in the case of cross metathesis), microwave power (W), reaction temperature (° C.), reaction time (h), percent conversion (%). Chromatographic purification conditions (isolated yield, %) are also listed where applicable.

Hydrogenation Procedures

Catalysts and Materials

Catalysts:

Palladium on charcoal (Pd/C) with 10% Pd concentration was used as supplied by Aldrich and stored in a desiccator. Tris(triphenylphosphine)rhodium(I) chloride (Wilkinson's catalyst, Rh(PPh₃)₃Cl) was used as supplied by Aldrich and stored in an argon or nitrogen filled dry box.

Gases:

High purity (<10 ppm oxygen) argon and hydrogen were supplied by BOC gases. Additional purification was achieved by passage of the gases through water, oxygen and hydrocarbon traps.

Solvents:

MeOH and DCM used in metal-catalysed hydrogenation reactions were degassed with high purity argon in addition to the general freeze-pump-thaw procedure (General Section) prior to use.

Reaction Vessels:

Fischer-Porter shielded aerosol pressure reactors (100 mL) fitted with pressure gauge heads and stirrer beads were employed.

Freeze-Pump-Thaw Procedure

Within a sealed vessel, a solvent or liquid reagent was immersed in liquid nitrogen and the contents frozen. The vessel was then evacuated to remove a majority of the gas, and resealed. Once back at ambient temperature, the solvent was refrozen and the vessel evacuated a second time. This procedure was repeated until gas evolution was no longer observed during the thaw cycle.

Conventional Hydrogenation

A Fischer-Porter tube was loaded with substrate, deoxygenated solvent, deoxygenated solvent additive and catalyst under an inert (argon or nitrogen) environment. The system was sealed and the pressure vessel then connected to a vacuum manifold and purged three times using vacuum and argon flushing cycles. After charging the vessel with hydrogen gas to the reported pressure, the reaction mixture was stirred at room temperature for a specified period of time and the reaction then terminated by venting the hydrogen gas. In the case of resin-bound substrates, reaction mixtures were filtered through a fritted syringe, washed with DCM (3-7 mL; 3×1 min), DMF (3-7 mL; 3×1 min) and MeOH (3-7 mL; 3×1 min), then dried in vacuo for 1 h prior to resin cleavage (General Section). For solution-based experiments, the catalyst was removed by filtration through a celite plug and the solvent evaporated under reduced pressure. Peptides were analysed by analytical RP-HPLC and mass spectrometry.

Microwave-Accelerated Hydrogenation

Microwave hydrogenation reactions were carried out on a CEM Discover™ system fitted with the Gas Addition™ option as described in the General Section. A microwave reactor vessel was loaded with substrate, deoxygenated solvent, deoxygenated solvent additive and catalyst in an inert (argon or nitrogen) environment. The system was sealed and the pressure vessel then connected to a vacuum manifold and purged three times using vacuum and argon flushing cycles. After charging the vessel with hydrogen gas to a reported pressure, the reaction mixture was irradiated with microwave energy whilst being stirred at the specified temperature for a specified period of time. Once cooled to room temperature, the hydrogen gas was vented from the system and catalyst deactivation was achieved by exposure to oxygen. In the case of resin-bound substrates, reaction mixtures were filtered through a fritted syringe, washed with DCM (3-7 mL; 3×1 min), DMF (3-7 mL; 3×1 min) and MeOH (3-7 mL; 3×1 min), then dried in vacuo for 1 h prior to resin cleavage (General Section). For solution-based experiments, the catalyst was removed by filtration through a celite plug and the solvent evaporated under reduced pressure. Peptides were analysed by RP-HPLC and mass spectrometry.

Hydrogenation experiments are described by the following format: Substrate (mmol), solvent (mL), additive (mL), catalyst (mol %), hydrogen pressure (psi), microwave power (W), reaction temperature (° C.) and reaction time (h).

c[A6,11]-Dicarba Human Insulin Transformations on the A-Chain Using Alternating SPPS-Catalysis des_(m-5)-[A6,11]-Agl-[A7]-Cys(^(t)Bu)-[A20]-Cys(Acm) Insulin A-chain 8

The automated, microwave-accelerated procedure outlined in the General Section was used for the synthesis of peptide 8 on Fmoc-Asn(Trt)-PEG-PS resin (1.18 g, 0.20 mmol). Quantities of HATU, DIPEA, piperidine and each Fmoc-amino acid were used as described by the automated protocols of the instrument and remained constant throughout this synthesis. The total amount of each coupling reagent and successive amino acid required, along with their reaction duration is summarised in the table below:—

TABLE Quantities of reagents and amino acids used in the synthesis of peptide 8 Total Mass (g) or Reaction Reagent volume (mL) Volume (mL) Time (min) 0.5M HATU in DMF 34 6.46 g — 2M DIPEA in NMP 17   5.9 mL — Fmoc-L-Asn(Trt)-OH 6 0.72 g 12 Fmoc-L-Agl-OH 11 0.74 g 12 Fmoc-L-Cys(Acm)-OH 6 0.50 g 12 Fmoc-L-Cys(^(t)Bu)—OH 6 0.48 g 12 Fmoc-L-Gln(Trt)-OH 6 0.73 g 12 Fmoc-L-Glu(O^(t)Bu)—OH 6 0.51 g 12 Fmoc-L-Ile-OH 6 0.42 g 12 Fmoc-L-Leu-OH 11 0.78 g 12 Fmoc-L-Ser(^(t)Bu)—OH 11 0.84 g 12 Fmoc-L-Thr(^(t)Bu)—OH 6 0.48 g 12 Fmoc-L-Tyr(^(t)Bu)—OH 11 1.01 g 12

After sequence completion, the resin-bound peptide was transferred into a fritted syringe and treated with an acetic anhydride solution (7 mL; DMF:acetic anhydride:NMM; 94:5:1) for 2 h. The resin was then washed with DMF (7 mL; 3×1 min), DCM (7 mL; 3×1 min) and MeOH (7 mL; 3×1 min), then left to dry in vacuo for 1 h. Prior to treatment with MeOH, a small aliquot of the resin-bound peptide was removed and subjected to Fmoc-deprotection in the presence of 20% v/v piperidine in DMF (1 mL; 1×1 min, 2×10 min), then washed with DMF (1 mL; 5×1 min), DCM (1 mL; 3×1 min) and MeOH (1 mL; 3×1 min). The aliquot of Fmoc-deprotected resin-tethered peptide was subjected to TFA-mediated cleavage (General Section) for RP-HPLC and mass spectral analysis. This supported formation of the desired peptide 8 in 80% purity. Mass spectrum (ESI⁺, MeCN:H₂O:HCOOH): m/z 986.5 [M+2H]²⁺, ½(C₈₇H₁₃₆N₂₀O₂₈S₂) requires 986.5. RP-HPLC (Agilent: Vydac C18 analytical column, 15→45% buffer B over 30 min): t_(R)=18.5 min.

des_(A1-5)-c[Δ⁴A6,11]-Dicarba-[A7]-Cys(^(t)Bu)-[20]-Cys(Acm) Insulin A-chain 10

Resin-bound peptide 8 was subjected to the general microwave-accelerated RCM procedure outlined in the General Section under the following conditions: Resin-bound 8 (1.82 g, 0.20 μmol), DCM (13 mL), 0.4 M LiCl in DMF (0.6 mL), 2^(nd) generation Grubbs' catalyst (34 mg, 40 μmol), 100 W μwave, 100° C., 2 h, 100% conversion into 10. Post metathesis, a small aliquot of resin-bound peptide was subjected to Fmoc-deprotection in the presence of 20% v/v piperidine in DMF (1 mL; 1×1 min, 2×10 min), then washed with DMF (1 mL; 5×1 min), DCM (1 mL; 3×1 min) and MeOH (1 mL; 3×1 min). The dried aliquot of Fmoc-deprotected resin-tethered peptide was subjected to TFA-mediated cleavage (General Section) for RP-HPLC and mass spectral analysis. This supported formation of the desired peptide as two isomers, 10(I) and 10(II), in a 75:25 ratio. 10(I): Mass spectrum (ESI⁺, MeCN:H₂O:HCOOH): m/z 972.6 [M+2H]²⁺, ½(C₈₅H₁₃₂N₂₀O₂₈S₂) requires 972.4. RP-HPLC (Agilent: Vydac C18 analytical column, 15→45% buffer B over 30 min): t_(R)=17.3 min. 10(11): Mass spectrum (ESI⁺, MeCN:H₂O:HCOOH): m/z 972.6 [M+2H]²⁺, ½(C₈₅H₁₃₂N₂₀O₂₈S₂) requires 972.4. RP-HPLC (Agilent: Vydac C18 analytical column, 15→45% buffer B over 30 min): t_(R)=17.5 min.

c[Δ⁴A6,11]-Dicarba-[A7]-Cys(^(t)Bu)-[A20]-Cys(Acm) human insulin A-chain 2

The automated, microwave-accelerated procedure outlined in the General Section was used to attach the remaining 5 residues on resin-bound peptide 10 (1.66 g, 0.20 μmol). Quantities of HATU, DIPEA, piperidine and each Fmoc-amino acid were used as described by the automated protocols of the instrument and remained constant throughout this synthesis. The total amount of each coupling reagent and successive amino acid required, along with their reaction duration is summarised in the table below:—

TABLE Quantities of reagents and amino acids used in the synthesis of peptide 2 Total Mass (g) or Reaction Reagent volume (mL) Volume (mL) Time (min) 0.5M HATU in DMF 11.0 2.10 g — 2M DIPEA in NMP 6.0   2.1 mL — Fmoc-L-Gly-OH 6.0 0.36 g 12 Fmoc-L-Gln(Trt)-OH 6.0 0.73 g 12 Fmoc-L-Glu(O^(t)Bu)—OH 6.0 0.51 g 12 Fmoc-L-Ile-OH 6.0 0.42 g 12 Fmoc-L-Val-OH 6.0 0.41 g 12

After sequence completion, the resin-bound peptide was transferred into a fritted syringe and subjected to Fmoc-deprotection in the presence of 20% v/v piperidine in DMF (7 mL; 1×1 min, 2×10 min). The resin was washed with DMF (7 mL; 5×1 min), DCM (7 mL; 3×1 min) and MeOH (7 mL; 3×1 min), then left to dry in vacuo for 1 h. The resin-bound peptide (1.50 g) was subjected to TFA-mediated cleavage (General Section) and RP-HPLC and mass spectral analysis of the resultant pale brown solid (241 mg) supported formation of the desired peptide as two isomers, 2(I) and 2(II), in a 75:25 ratio. 2(I): Mass spectrum (ESI⁺, MeCN:H₂O:HCOOH): m/z 824.2 [M+3H]³⁺, ⅓(C₁₀₈H₁₇₁N₂₆O₃₆S₂) requires 824.1; 1235.6 [M+2H]²⁺, ½(C₁₀₈H₁₇₀N₂₆O₃₆S₂) requires 1235.6. RP-HPLC (Agilent: Vydac C18 analytical column, 15→45% buffer B over 30 min): t_(R)=18.2 min. 2(II): Mass spectrum (ESI⁺, MeCN:H₂O:HCOOH): m/z 824.3 [M+3H]³⁺, ⅓(C₁₀₈H₁₇₁N₂₆O₃₆S₂) requires 824.1; 1235.6 [M+2H]²⁺, ½(C₁₀₈H₁₇₁N₂₆O₃₆S₂) requires 1235.6. RP-HPLC (Agilent: Vydac C18 analytical column, 15→45% buffer B over 30 min): t_(R)=19.2 min.

des_(A1-5)-c[A6,11]-Dicarba-[A7]-Cys(^(t)Bu)-[A20]-Cys(Acm) Insulin A-Chain 12

-   -   12

Resin-bound peptide 10 was subjected to the general microwave-accelerated hydrogenation procedure outlined in the General Section under the following conditions: Resin-bound 10 (792 mg, 100 μmol), DCM (4.5 mL), MeOH (0.5 mL), Wilkinsons catalyst, H₂ (90 psi), 100 W pwave, 100° C., 2 h, 95% conversion into 12. Post metathesis, a small aliquot of resin-bound peptide was subjected to Fmoc-deprotection in the presence of 20% v/v piperidine in DMF (1 mL; 1×1 min, 2×10 min), then washed with DMF (1 mL; 5×1 min), DCM (1 mL; 3×1 min) and MeOH (1 mL; 3×1 min). The dried aliquot of Fmoc-deprotected resin-tethered peptide was subjected to TFA-mediated cleavage (General Section) for RP-HPLC and mass spectral analysis. This supported formation of the saturated carbocycle 12. Mass spectrum (ESI⁺, MeCN:H₂O:HCOOH): m/z 973.6 [M+2H]²⁺, ½(C₈₅H₁₃₄N₂₀O₂₈S₂) requires 973.5. RP-HPLC (Agilent: Vydac C18 analytical column, 15→45% buffer B over 30 min): t_(R)=18.2 min.

c[A6,11]-Dicarba-[A7]-Cys(^(t)Bu)-[A20]-Cys(Acm) Insulin A-Chain 4

The automated, microwave-accelerated procedure outlined in the General Section was used to attach the remaining 5 residues on resin-bound peptide 12 (1.69 g, 200 pmol). Quantities of HATU, DIPEA, piperidine and each Fmoc-amino acid were used as described by the automated protocols of the instrument and remained constant throughout this synthesis. The total amount of each coupling reagent and successive amino acid required, along with their reaction duration is summarised in the table below:—

TABLE Quantities of reagents and amino acids used in the synthesis of peptide 4 Total Mass (g) or Reaction Reagent volume (mL) Volume (mL) Time (min) 0.5M HATU in DMF 11.0 2.10 g — 2M DIPEA in NMP 6.0   2.1 mL — Fmoc-L-Gly-OH 6.0 0.36 g 12 Fmoc-L-Gln(Trt)-OH 6.0 0.73 g 12 Fmoc-L-Glu(O^(t)Bu)—OH 6.0 0.51 g 12 Fmoc-L-Ile-OH 6.0 0.42 g 12 Fmoc-L-Val-OH 6.0 0.41 g 12

After sequence completion, the resin-bound peptide was transferred into a fritted syringe and subjected to Fmoc-deprotection in the presence of 20% v/v piperidine in DMF (7 mL; 1×1 min, 2×10 min). The resin was washed with DMF (7 mL; 5×1 min), DCM (7 mL; 3×1 min) and MeOH (7 mL; 3×1 min), then left to dry in vacuo for 1 h. The resin-bound peptide (1.51 g) was subjected to TFA-mediated cleavage (General Section) and RP-HPLC and mass spectral analysis of the isolated pale brown solid (220 mg) supported formation of the desired peptide 4 in 70% purity. Mass spectrum (ESI⁺, MeCN:H₂O:HCOOH): m/z 824.7 [M+3H]³⁺, ⅓(C₁₀₈H₁₇₃N₂₆O₃₆S₂) requires 824.7; 1236.8 [M+2H]²⁺, ½(C₁₀₈H₁₇₂N₂₆O₃₆S₂) requires 1236.6. RP-HPLC (Agilent: Vydac C18 analytical column, 15→45% buffer B over 30 min): t_(R)=18.7 min.

des_(A1-5)-[6,11]-Agl-[A7]-Pre-[A20]-Cys(Acm) Insulin A-Chain 9

The automated, microwave-accelerated procedure outlined in the General Section was used for the synthesis of peptide on Fmoc-Asn(Trt)-PEG-PS resin (667 mg, 100 pmol). Quantities of HATU, DIPEA, piperidine and each Fmoc-amino acid were used as described by the automated protocols of the instrument and remained constant throughout this synthesis. The total amount of each coupling reagent and successive amino acid required, along with their reaction duration is summarised in the table below:—

TABLE Quantities of reagents and amino acids used in the synthesis of peptide 9 Total Mass (g) or Reaction Reagent volume (mL) Volume (mL) Time (min) 0.5M HATU in DMF 17.0  3.24 g — 2M DIPEA in NMP 9.0    3.1 mL — Fmoc-L-Asn(Trt)-OH 3.0 0.358 g 12 Fmoc-L-Agl-OH 6.0 0.405 g 12 Fmoc-L-Cys(Acm)-OH 3.0 0.249 g 12 Fmoc-L-Gln(Trt)-OH 3.0 0.367 g 12 Fmoc-L-Glu(O^(t)Bu)—OH 3.0 0.256 g 12 Fmoc-L-Ile-OH 3.0 0.212 g 12 Fmoc-L-Leu-OH 6.0 0.424 g 12 Fmoc-L-Pre-OH 3.0 0.219 g 12 Fmoc-L-Ser(^(t)Bu)—OH 6.0 0.460 g 12 Fmoc-L-Thr(^(t)Bu)—OH 3.0 0.238 g 12 Fmoc-L-Tyr(^(t)Bu)—OH 6.0 0.551 g 12

After sequence completion, the resin-bound peptide was transferred into a fritted syringe and treated with an acetic anhydride solution (7 mL; DMF:acetic anhydride:NMM; 94:5:1) for 2 h. The resin was then washed with DMF (7 mL; 3×1 min), DCM (7 mL; 3×1 min) and MeOH (7 mL; 3×1 min), then left to dry in vacuo for 1 h. Prior to treatment with MeOH, a small aliquot of the resin-bound peptide was removed and subjected to Fmoc-deprotection in the presence of 20% v/v piperidine in DMF (1 mL; 1×1 min, 2×10 min), then washed with DMF (1 mL; 5×1 min), DCM (1 mL; 3×1 min) and MeOH (1 mL; 3×1 min). The aliquot of Fmoc-deprotected resin-tethered peptide was subjected to TFA-mediated cleavage (General Section) for RP-HPLC and mass spectral analysis. This supported formation of the desired peptide 9 in 85% purity. Mass spectrum (ESI⁺, MeCN:H₂O:HCOOH): m/z 970.1 [M+2H]²⁺, ½(C₈₇H₁₃₄N₂₀O₂₈S) requires 969.5; 978.7 [M+H₂O+2H]²⁺, ½(C₈₇H₁₃₆N₂₀O₂₉S) requires 978.5; 1026.8 [M+TFA+2H]²⁺, ½(C₈₉H₁₃₅F₃N₂₀O₃₀S) requires 1026.5. RP-HPLC (Agilent: Vydac C18 analytical column, 15→45% buffer B over 30 min): t_(R)=21.6 min.

des_(A1-5)-c[Δ⁴A6,11]-Dicarba-[A7]-Pre-[A20]-Cys(Acm) Insulin A-Chain 11

Resin-bound peptide 9 was subjected to the microwave-accelerated RCM procedure outlined in the General Section under the following conditions: Resin-bound 9 (801 mg, 100 pmol), DCM (6 mL), 0.4 M LiCl in DMF (0.2 mL), 2^(nd) generation Grubbs' catalyst (17 mg, 20 pmol), 100 W pwave, 100° C., 2 h, 95% conversion into 11. Post metathesis, a small aliquot of resin-bound peptide was subjected to Fmoc-deprotection in the presence of 20% v/v piperidine in DMF (1 mL; 1×1 min, 2×10 min), then washed with DMF (1 mL; 5×1 min), DCM (1 mL; 3×1 min) and MeOH (1 mL; 3×1 min). The aliquot of Fmoc-deprotected resin-tethered peptide was subjected to TFA-mediated cleavage (General Section) for RP-HPLC and mass spectral analysis. This supported formation of the desired peptide as two isomers, 11(I) and 11(II), in a 3:7 ratio. 11(I): Mass spectrum (ESI⁺, MeCN:H₂O:HCOOH): m/z 955.5 [M+2H]²⁺, ½(C₈₅H₁₃₀N₂₀O₂₈S) requires 955.5; 964.3 [M+H₂O+2H]²⁺, ½(C₈₅H₁₃₂N₂₀O₂₉S) requires 964.5; 1013.0 [M+TFA+2H]²⁺, ½(C₈₇H₁₃₁F₃N₂₀O₃₀S) requires 1012.5. RP-HPLC (Agilent: Vydac C18 analytical column, 15→45% buffer B over 30 min): t_(R)=21.3 min. 11(II): Mass spectrum (ESI⁺, MeCN:H₂O:HCOOH): m/z 955.5 [M+2H]²⁺, ½(C₈₅H₁₃₀N₂₀O₂₈S) requires 955.5; 964.2 [M+H₂O+2H]²⁺, ½(C₈₅H₁₃₂N₂₀O₂₉S) requires 964.5; 1012.6 [M+TFA+2H]²⁺, ½(C₈₇H₁₃₁F₃N₂₀O₃₀S) requires 1012.5. RP-HPLC (Agilent: Vydac C18 analytical column, 15→45% buffer B over 30 min): t_(R)=21.7 min.

des_(A1-5)-c[A6,11]-Dicarba-[A7]-Pre-[A20]-Cys(Acm) Insulin A-Chain 13

Resin-bound peptide 11 was subjected to the microwave-accelerated hydrogenation procedure outlined in the General Section under the following conditions: Resin-bound 11 (432 mg, 60 μmol), DCM (4.5 mL), MeOH (0.5 mL), Wilkinsons catalyst, H₂ (80 psi), 80 W pwave, 70° C., 2 h, 100% conversion into 13. Post metathesis, a small aliquot of resin-bound peptide was subjected to Fmoc-deprotection in the presence of 20% v/v piperidine in DMF (1 mL; 1×1 min, 2×10 min), then washed with DMF (1 mL; 5×1 min), DCM (1 mL; 3×1 min) and MeOH (1 mL; 3×1 min). The aliquot of Fmoc-deprotected resin-tethered peptide was subjected to TFA-mediated cleavage (General Section) for RP-HPLC and mass spectral analysis. This supported formation of the saturated carbocycle 13 and 10% over reduction to cyclic peptide 22. Mass spectrum (ESI⁺, MeCN:H₂O:HCOOH): m/z 957.5 [M₂₂+2H]²⁺; 965.6 [M₁₃+H₂O+2H]²⁺, ½(C₈₅H₁₃₄N₂₀O₂₉S) requires 965.5; 1013.5 [M₁₃+TFA+2H]²⁺, ½(C₈₇H₁₃₃F₃N₂₀O₃₀S) requires 1013.5. RP-HPLC (Agilent: Vydac C18 analytical column, 15→45% buffer B over 30 min): t_(R)=21.6 min.

des_(A1-5)-c[6,11]-Dicarba-[7]-Crt-[20]-Cys(Acm) Insulin A-Chain 14

Method A:

Resin-bound Fmoc-protected peptide 13 was subjected to the general conventional cross metathesis procedure outlined in the General Section under the following conditions: 13 (216 mg, 30 pmol), DCM (4 mL), 2^(nd) generation Hoveyda-Grubbs' catalyst (3.8 mg, 6 μmol), 0.4 M LiCl in DMF (0.2 mL), cis-2-butene (12 psi), Δ, 84 h, 0% conversion into 14. Post metathesis, a small aliquot of resin-bound peptide was subjected to Fmoc-deprotection in the presence of 20% v/v piperidine in DMF (4 mL; 1×1 min, 2×10 min), then washed with DMF (4 mL; 5×1 min), DCM (4 mL; 3×1 min) and MeOH (4 mL; 3×1 min). The aliquot of Fmoc-deprotected resin-tethered peptide was subjected to TFA-mediated cleavage (General Section) for RP-HPLC and mass spectral analysis. This supported only starting material. Mass spectrum (ESI⁺, MeCN:H₂O:HCOOH): m/z 957.5, 965.7, 1013.5.

Method B:

Resin-bound peptide 13 was subjected to the microwave-accelerated CM procedure outlined in the General Section under the following conditions: Resin-bound 13 (216 mg, 30 μmol), DCM (4 mL), 0.4 M LiCl in DMF (0.2 mL), 2^(nd) generation Hoveyda-Grubbs' catalyst (3.8 mg, 6 μmol), cis-2-butene (12 psi), 80 W pwave, 70° C., 4 h, 95% conversion into 14. After the first 2 h, the reaction mixture was purge-charged with argon to remove any volatile by-products and re-filled with butene for the second 2 h reaction duration. Post metathesis, a small aliquot of resin-bound peptide was subjected to Fmoc-deprotection in the presence of 20% v/v piperidine in DMF (1 mL; 1×1 min, 2×10 min), then washed with DMF (1 mL; 5×1 min), DCM (1 mL; 3×1 min) and MeOH (1 mL; 3×1 min). The aliquot of Fmoc-deprotected resin-tethered peptide was subjected to TFA-mediated cleavage (General Section) for RP-HPLC and mass spectral analysis. This supported formation of the desired peptide 14. Trace amounts of hydrated starting materials and over reduced peptide were also detected in the mass spectrum. Mass spectrum (ESL MeCN:H₂O:HCOOH): m/z 949.7 [M₁₄+2H]²⁺, ½(C₈₄H₁₃₀N₂₀O₂₈S) requires 949.5; 957.7 [M₂₂+2H]²⁺; 965.7 [M₁₃+H₂O+2H]²⁺; 1013.6 [M₁₃+TFA+2H]²⁺. RP-HPLC (Agilent: Vydac C18 analytical column, 15→45% buffer B over 30 min): t_(R(14))=16.2 min.

Cross Metathesis of the Activated A-Chain 14 and Ac-D,L-Agl-OMe→15

Resin-bound Fmoc-protected peptide 14 was subjected to the general microwave-accelerated cross metathesis procedure outlined in the General Section under the following conditions: Resin-bound 14 (207 mg, 30 μmol), DCM (4 mL), 0.4 M LiCl in DMF (0.2 mL), 2^(nd) generation Hoveyda Grubbs' catalyst (3.8 mg, 6 μmol), Ac-D,L-Agl-OMe (51 mg, 300 μmol) 100 W pwave, 100° C., 2 h, 90% conversion into 15. Post metathesis, a small aliquot of resin-bound peptide was subjected to Fmoc-deprotection in the presence of 20% v/v piperidine in DMF (1 mL; 1×1 min, 2×10 min), then washed with DMF (1 mL; 5×1 min), DCM (1 mL; 3×1 min) and MeOH (1 mL; 3×1 min). The aliquot of Fmoc-deprotected resin-tethered peptide was subjected to TFA-mediated cleavage (General Section) for RP-HPLC and mass spectral analysis. This supported formation of the desired peptide 15. Mass spectrum (ESI⁺, MeCN:H₂O:HCOOH): m/z 1014.1 [M₁₅+2H]²⁺, ½(C₈₃H₁₃₇N₂₁O₃₁S) requires 1014.0, 964.8 [M₁₃+2H]²⁺, 957.4 [M₂₂+2H]²⁺.

Addition of GIVEQ to Peptide 15

The automated, microwave-accelerated procedure outlined in the General Section was used to attach the remaining five residues (GIVEQ) onto resin-bound peptide 15 (200 mg, 30 pmol). Quantities of HATU, DIPEA and each Fmoc-amino acid were used as described by the automated protocols of the instrument and remained constant throughout this synthesis. The total amount of each coupling reagent and successive amino acid required, along with their reaction duration is summarised in Table below:—

TABLE Quantities of reagents and amino acids used in the synthesis of peptide 16 Total Mass (g) or Reaction Reagent volume (mL) Volume (mL) Time (min) 0.5M HATU in DMF 6.0  1.14 g — 2M DIPEA in NMP 3.0    1.0 mL — Fmoc-L-Gly-OH 3.0 0.178 g 12 Fmoc-L-Gln(Trt)-OH 3.0 0.367 g 12 Fmoc-L-Glu(OtBu)—OH 3.0 0.256 g 12 Fmoc-L-Ile-OH 3.0 0.212 g 12 Fmoc-L-Val-OH 3.0 0.204 g 12

After sequence completion, the resin-bound peptide was transferred into a fritted syringe and subjected to Fmoc-deprotection in the presence of 20% v/v piperidine in DMF (4 mL; 1×1 min, 2×10 min). The resin was washed with DMF (4 mL; 5×1 min), DCM (4 mL; 3×1 min) and MeOH (4 mL; 3×1 min), left to dry in vacuo for 1 h, and subjected to TFA-mediated cleavage (General Section) for RP-HPLC and mass spectral analysis. This supported formation of the desired peptide 16 and trace amounts of the over reduced peptide 23. Mass spectrum (ESI⁺, MeCN:H₂O:HCOOH): m/z 1277.0 [M₁₆+2H]²⁺, ½(C₁₁₂H₁₇₅N₂₇O₃₉S) requires 1277.1; 1220.9 [M₂₃+2H]²⁺. RP-HPLC (Agilent: Vydac C18 analytical column, 15→45% buffer B over 30 min): t_(R(16))=19.3 min.

Preparation of Dicarba Analogues of Rapid and Long Acting Insulins des_(A1-5)-[A6,11]-Agl-[A7]-Cys(^(t)Bu)-[A20]-Cys(Acm) Human Insulin Glargine A-chain 40

The automated, microwave-accelerated procedure outlined in the General Section was used for the synthesis of peptide 40 on Fmoc-Gly-PEG-PS resin (1.10 g, 200 μmol). Quantities of HATU, DIPEA, piperidine and each Fmoc-amino acid were used as described by the automated protocols of the instrument and remained constant throughout this synthesis. The total amount of each coupling reagent and successive amino acid required, along with their reaction duration is summarised in the table below:—

TABLE Quantities of reagents and amino acids used in the synthesis of peptide 40 Total Mass (g) or Reaction Reagent volume (mL) Volume (mL) Time (min) 0.5M HATU in DMF 17.0  3.24 g — 2M DIPEA in NMP 9.0    3.1 mL — Fmoc-L-Asn(Trt)-OH 3.0 0.358 g 12 Fmoc-L-Agl-OH 6.0 0.405 g 12 Fmoc-L-Cys(Acm)-OH 3.0 0.249 g 12 Fmoc-L-Cys(^(t)Bu)—OH 3.0 0.240 g 12 Fmoc-L-Gln(Trt)-OH 3.0 0.367 g 12 Fmoc-L-Glu(O^(t)Bu)—OH 3.0 0.256 g 12 Fmoc-L-Ile-OH 3.0 0.212 g 12 Fmoc-L-Leu-OH 6.0 0.424 g 12 Fmoc-L-Ser(^(t)Bu)—OH 6.0 0.460 g 12 Fmoc-L-Thr(^(t)Bu)—OH 3.0 0.238 g 12 Fmoc-L-Tyr(^(t)Bu)—OH 6.0 0.551 g 12

After sequence completion, the resin-bound peptide was transferred into a fritted syringe and treated with an acetic anhydride solution (7 mL; DMF:acetic anhydride:NMM; 94:5:1) for 2 h. The resin was then washed with DMF (7 mL; 3×1 min), DCM (7 mL; 3×1 min) and MeOH (7 mL; 3×1 min), then left to dry in vacuo for 1 h. Prior to treatment with MeOH, a small aliquot of the resin-bound peptide was removed and subjected to Fmoc-deprotection in the presence of 20% v/v piperidine in DMF (1 mL; 1×1 min, 2×10 min), then washed with DMF (1 mL; 5×1 min), DCM (1 mL; 3×1 min) and MeOH (1 mL; 3×1 min). The aliquot of Fmoc-deprotected resin-tethered peptide was subjected to TFA-mediated cleavage (General Section) for RP-HPLC and mass spectral analysis. This supported formation of the desired peptide 40 in 70% purity. Mass spectrum (ESI⁺, MeCN:H₂O:HCOOH): m/z 958.3 [M+2H]²⁺, ½(C₈₅H₁₃₃N₁₉O₂₇S₂) requires 958.0. RP-HPLC (Agilent: Vydac C18 analytical column, 15→45% buffer B over 30 min): t_(R)=18.9 min.

des_(A1-5)-c[Δ⁴A6,11]-Dicarba-[A7]-CyseBu[A20]-Cys(Acm) Human Insulin Glargine A-Chain 41

Resin-bound peptide 40 was subjected to the general microwave-accelerated RCM procedure outlined in the General Section under the following conditions: Resin-bound 40 (1.35 g, 0.20 mmol), DCM (13 mL), 0.4 M LiCl in DMF (0.5 mL), 2^(nd) generation Grubbs' catalyst (34 mg, 40 μmol), 100 W pwave, 100° C., 4 h, 100% conversion into 41. Post metathesis, a small aliquot of resin-bound peptide was subjected to Fmoc-deprotection in the presence of 20% v/v piperidine in DMF (1 mL; 1×1 min, 2×10 min), then washed with DMF (1 mL; 5×1 min), DCM (1 mL; 3×1 min) and MeOH (1 mL; 3×1 min). The aliquot of Fmoc-deprotected resin-tethered peptide was subjected to TFA-mediated cleavage (General Section) for RP-HPLC and mass spectral analysis. This supported formation of the desired peptide as two isomers, 41(I) and 41(II), in a 7:3 ratio. 41(I): Mass spectrum (ESI⁺, MeCN:H₂O:HCOOH): m/z 944.0 [M+2H]²⁺, ½(C₈₃H₁₂₉N₁₉O₂₇S₂) requires 943.9. RP-HPLC (Agilent: Vydac C18 analytical column, 15→45% buffer B over 30 min): t_(R)=17.8 min. 41(II): Mass spectrum (ESI⁺, MeCN:H₂O:HCOOH): m/z 944.2 [M+2H]²⁺, ½(C₈₃H₁₂₉N₁₉O₂₇S₂) requires 943.9. RP-HPLC (Agilent: Vydac C18 analytical column, 15→45% buffer B over 30 min): t_(R)=17.9 min.

des_(A1-5)-c[A6,11]-Dicarba-[A7]-Cys(^(t)Bu)-[A20]-Cys(Acm) Human Insulin Glargine A-Chain 42

Resin-bound peptide 41 was subjected to the microwave-accelerated hydrogenation procedure outlined in the General Section under the following conditions: Resin-bound 41 (0.52 g, 0.20 mmol), DCM (4.5 mL), MeOH (0.5 mL), Wilkinsons catalyst, H₂ (80 psi), 80 W pwave, 80° C., 4 h, 100% conversion into 42. Post metathesis, a small aliquot of resin-bound peptide was subjected to Fmoc-deprotection in the presence of 20% v/v piperidine in DMF (1 mL; 1×1 min, 2×10 min), then washed with DMF (1 mL; 5×1 min), DCM (1 mL; 3×1 min) and MeOH (1 mL; 3×1 min). The aliquot of Fmoc-deprotected resin-tethered peptide was subjected to TFA-mediated cleavage (General Section) for RP-HPLC and mass spectral analysis. This supported formation of the saturated carbocycle 42. Mass spectrum (ESI⁺, MeCN:H₂O:HCOOH): m/z 945.0 [M+2H]²⁺, ½(C₈₃H₁₃₁N₁₉O₂₇S₂) requires 944.9. RP-HPLC (Agilent: Vydac C18 analytical column, 15→45% buffer B over 30 min): t_(R)=18.7 min.

c[Δ⁴A6,11]-Dicarba-[A7]-Cys(^(t)Bu)-[A20]-Cys(Acm) Human Insulin Glargine A-Chain 43

The automated, microwave-accelerated procedure outlined in the General Section was used to attach the remaining 5 residues on resin-bound peptide 41 (1.16 g, 0.20 mmol). Quantities of HATU, DIPEA, piperidine and each Fmoc-amino acid were used as described by the automated protocols of the instrument and remained constant throughout this synthesis. The total amount of each coupling reagent and successive amino acid required, along with their reaction duration is summarised in the table below:—

TABLE Quantities of reagents and amino acids used in the synthesis of peptide 43 Total Mass (g) or Reaction Reagent volume (mL) Volume (mL) Time (min) 0.5M HATU in DMF 11.0  2.10 g — 2M DIPEA in NMP 6.0    2.1 mL — Fmoc-L-Gly-OH 6.0 0.357 g 12 Fmoc-L-Gln(Trt)-OH 6.0 0.733 g 12 Fmoc-L-Glu(O^(t)Bu)—OH 6.0 0.511 g 12 Fmoc-L-Ile-OH 6.0 0.424 g 12 Fmoc-L-Val-OH 6.0 0.407 g 12

After sequence completion, the resin-bound peptide was transferred into a fritted syringe and subjected to Fmoc-deprotection in the presence of 20% v/v piperidine in DMF (7 mL; 1×1 min, 2×10 min). The resin was washed with DMF (7 mL; 5×1 min), DCM (7 mL; 3×1 min) and MeOH (7 mL; 3×1 min), then left to dry in vacuo for 1 h. The resin-bound peptide (0.98 g) was subjected to TFA-mediated cleavage (General Section), and RP-HPLC and mass spectral analysis of the resultant pale brown solid (300 mg) supported formation of the desired peptide as two isomers, 43(1) and 43(11), in a 7:3 ratio. 43(1): Mass spectrum (ESI⁺, MeCN:H₂O:HCOOH): m/z 1207.2 [M+2H]²⁺, ½(C₁₀₆H₁₆₇N₂₅O₃₅S₂) requires 1207.1. RP-HPLC (Agilent: Vydac C18 analytical column, 15→45% buffer B over 30 min): t_(R)=18.6 min. 43(II): Mass spectrum (ESI⁺, MeCN:H₂O:HCOOH): m/z 1207.2 [M+2H]²⁺, ½(C₁₀₆H₁₆₇N₂₅O₃₅S₂) requires 1207.1. RP-HPLC (Agilent: Vydac C18 analytical column, 15→45% buffer B over 30 min): t_(R)=19.5 min.

c[A6,11]-Dicarba-[A7]-Cys(^(t)Bu)-[A20]-Cys(Acm) Human Insulin Glargine A-Chain 4

The automated, microwave-accelerated procedure outlined in the General Section was used to attach the remaining 5 residues on resin-bound peptide 42 (0.55 g, 0.10 mmol). Quantities of HATU, DIPEA, piperidine and each Fmoc-amino acid were used as described by the automated protocols of the instrument and remained constant throughout this synthesis. The total amount of each coupling reagent and successive amino acid required, along with their reaction duration is summarised in the table below:—

TABLE Quantities of reagents and amino acids used in the synthesis of peptide 4 Total Mass (g) or Reaction Reagent volume (mL) Volume (mL) Time (min) 0.5M HATU in DMF 6.0  1.14 g — 2M DIPEA in NMP 3.0    1.0 mL — Fmoc-L-Gly-OH 3.0 0.178 g 12 Fmoc-L-Gln(Trt)-OH 3.0 0.367 g 12 Fmoc-L-Glu(O^(t)Bu)—OH 3.0 0.256 g 12 Fmoc-L-Ile-OH 3.0 0.212 g 12 Fmoc-L-Val-OH 3.0 0.204 g 12

After sequence completion, the resin-bound peptide was transferred into a fritted syringe and subjected to Fmoc-deprotection in the presence of 20% v/v piperidine in DMF (7 mL; 1×1 min, 2×10 min). The resin was washed with DMF (7 mL; 5×1 min), DCM (7 mL; 3×1 min) and MeOH (7 mL; 3×1 min), then left to dry in vacuo for 1 h. The resin-bound peptide (0.49 g) was subjected to TFA-mediated cleavage (General Section), and RP-HPLC and mass spectral analysis of the resultant solid (100 mg) supported formation of the desired peptide 4 in 45% purity. Mass spectrum (ESI⁺, MeCN:H₂O:HCOOH): m/z 805.6 [M+H]⁺, C₁₀₆H₁₇₀N₂₅O₃₅S₂ requires 805.7; 1208.4 [M+2H]²⁺, ½(C₁₀₆H₁₆₉N₂₅O₃₅S₂) requires 1208.1. RP-HPLC (Agilent: Vydac C18 analytical column, 15→45% buffer B over 30 min): t_(R)=19.2 min.

des_(A1-5)-[A6,11]-Agl-[A7]-Cys(^(t)Bu)-[A20]-Cys(Acm)-[A21]-Asp Human Insulin A-Chain 45

Esterification of Fmoc-L-Asp(O^(t)Bu)-OH (123 mg, 300 μmol) on Wang resin (91 mg, 100 μmol) was performed according to the procedure described in the General Section using DIC (47 μL, 300 μmol) and DMAP (3.7 mg, 30 μmol) for 16 h. The automated microwave-accelerated procedure was then used for the synthesis of peptide 45 on Fmoc-Asp(O′Bu)-Wang resin (128 mg, 100 μmol). Quantities of HATU, DIPEA, piperidine and each Fmoc-amino acid were used as described by the automated protocols of the instrument and remained constant throughout this synthesis. The total amount of each coupling reagent and successive amino acid required, along with their reaction duration is summarised in the table below:—

TABLE Quantities of reagents and amino acids used in the synthesis of peptide 45 Total Mass (g) or Reaction Reagent volume (mL) Volume (mL) Time (min) 0.5M HATU in DMF 17.0 3.24 g — 2M DIPEA in NMP 9.0   3.1 mL — Fmoc-L-Asn(Trt)-OH 3.0 0.36 g 12 Fmoc-L-Agl-OH 6.0 0.41 g 12 Fmoc-L-Cys(Acm)-OH 3.0 0.25 g 12 Fmoc-L-Cys(^(t)Bu)—OH 3.0 0.24 g 12 Fmoc-L-Gln(Trt)-OH 3.0 0.37 g 12 Fmoc-L-Glu(O^(t)Bu)—OH 3.0 0.26 g 12 Fmoc-L-Ile-OH 3.0 0.21 g 12 Fmoc-L-Leu-OH 6.0 0.42 g 12 Fmoc-L-Ser(^(t)Bu)—OH 6.0 0.46 g 12 Fmoc-L-Thr(^(t)Bu)—OH 3.0 0.24 g 12 Fmoc-L-Tyr(^(t)Bu)—OH 6.0 0.55 g 12

After sequence completion, the resin-bound peptide was transferred into a fritted syringe and treated with an acetic anhydride solution (4 mL; DMF:acetic anhydride: NMM; 94:5:1) for 2 h. The resin was washed with DMF (4 mL; 3×1 min), DCM (4 mL; 3×1 min) and MeOH (4 mL; 3×1 min), then left to dry in vacuo for 1 h. Prior to treatment with MeOH, a small aliquot of the resin-bound peptide was removed and subjected to Fmoc-deprotection in the presence of 20% v/v piperidine in DMF (1 mL; 1×1 min, 2×10 min), and washed with DMF (1 mL; 5×1 min), DCM (1 mL; 3×1 min) and MeOH (1 mL; 3×1 min). The dried aliquot of Fmoc-deprotected resin-tethered peptide was subjected to TFA-mediated cleavage (General Section) and RP-HPLC and mass spectral analysis of the resultant solid supported formation of the desired peptide 45 in 70% purity. Mass spectrum (ESI⁺, MeCN:H₂O:HCOOH): m/z 987.1 [M+2H]²⁺, ½(C₈₇H₁₃₅N₁₉O₂₉S₂) requires 987.0. RP-HPLC (Agilent: Vydac C18 analytical column, 15→45% buffer B over 30 min): t_(R)=19.1 min.

des_(A1-5)-c[Δ⁴A6,11]-Dicarba-[A7]-Cys(^(t)Bu)-[A20]-Cys(Acm)-[A21]-Asp Human Insulin A-Chain 46

Resin-bound peptide 45 was subjected to the general microwave-accelerated RCM procedure outlined in the General Section under the following conditions: Resin-bound 45 (368 mg, 100 μmol), DCM (4.75 mL), 0.4 M LiCl in DMF (0.25 mL), 2^(nd) generation Grubbs' catalyst (17 mg, 20 μmol), 100 W pwave, 100° C., 2 h, 100% conversion into 46. Post metathesis, a small aliquot of resin-bound peptide was subjected to Fmoc-deprotection in the presence of 20% v/v piperidine in DMF (1 mL; 1×1 min, 2×10 min), then washed with DMF (1 mL; 5×1 min), DCM (1 mL; 3×1 min) and MeOH (1 mL; 3×1 min). The dried aliquot of Fmoc-deprotected resin-tethered peptide was subjected to TFA-mediated cleavage (General Section) for RP-HPLC and mass spectral analysis. This supported formation of the desired cyclic peptide 46. Mass spectrum (ESI⁺, MeCN:H₂O:HCOOH): m/z 973.0 [M+2H]²⁺, ½(C₈₅H₁₃₁N₁₉O₂₉S2) requires 972.9. RP-HPLC (Agilent: Vydac C18 analytical column, 15→45% buffer B over 30 min): t_(R)=18.1 min.

c[Δ⁴A6,11]-Dicarba[A7]-Cys(^(t)Bu)-[A20]-Cys(Acm)-[A21]-Asp Human Insulin A-Chain 47

The automated, microwave-accelerated procedure outlined in the General Section was used to attach the remaining 5 residues on resin-bound peptide 46 (351 g, 100 μmol). Quantities of HATU, DIPEA, piperidine and each Fmoc-amino acid were used as described by the automated protocols of the instrument and remained constant throughout this synthesis. The total amount of each coupling reagent and successive amino acid required, along with their reaction duration is summarised in the table below:—

TABLE Quantities of reagents and amino acids used in the synthesis of peptide 47 Total Mass (g) or Reaction Reagent volume (mL) Volume (mL) Time (min) 0.5M HATU in DMF 6.0  1.14 g — 2M DIPEA in NMP 3.0    1.0 mL — Fmoc-L-Gly-OH 3.0 0.178 g 12 Fmoc-L-Gln(Trt)-OH 3.0 0.367 g 12 Fmoc-L-Glu(O^(t)Bu)—OH 3.0 0.256 g 12 Fmoc-L-Ile-OH 3.0 0.212 g 12 Fmoc-L-Val-OH 3.0 0.204 g 12

After sequence completion, the resin-bound peptide was transferred into a fritted syringe and subjected to Fmoc-deprotection in the presence of 20% v/v piperidine in DMF (7 mL; 1×1 min, 2×10 min). The resin was washed with DMF (7 mL; 5×1 min), DCM (7 mL; 3×1 min) and MeOH (7 mL; 3×1 min), then left to dry in vacuo for 1 h. The resin-bound peptide (359 g) was subjected to TFA-mediated cleavage (General Section) and RP-HPLC and mass spectral analysis of the resultant pale brown solid (260 mg) supported formation of the desired peptide as two isomers, 47(1) and 47(11), in a 73:27 ratio. 47(1): Mass spectrum (ESI⁺, MeCN:H₂O:HCOOH): m/z 824.8 [M+3H]³⁺, ⅓(C₁₀₈H₁₇₀N₂₅O₃₇S₂) requires 824.4; 1236.8 [M+2H]²⁺, ½(C₁₀₈H₁₆₉N₂₅O₃₇S₂) requires 1236.0. RP-HPLC (Agilent: Vydac C18 analytical column, 15→45% buffer B over 30 min): t_(R)=18.8 min. 47(II): Mass spectrum (ESI⁺, MeCN:H₂O:HCOOH): m/z 824.8 [M+3H]³⁺, ⅓(C₁₀₈H₁₇₀N₂₅O₃₇S₂) requires 824.4; 1236.8 [M+2H]²⁺, ½(C₁₀₈H₁₆₉N₂₅O₃₇S₂) requires 1236.0. RP-HPLC (Agilent: Vydac C18 analytical column, 15→45% buffer B over 30 min): t_(R)=19.6 min.

des_(A1-5)-[A6,11]-Agl-[A7]-Cys(^(t)Bu)-[A20]-Cys(Acm)-[A21]-β-Asn Human Insulin A-Chain 48

Attachment of Fmoc-L-Asp-O^(t)Bu (123 mg, 300 μmol) on Rink Amide resin (250 mg, 100 μmol) was performed according to the procedure described in the General Section using HATU (114 mg, 300 μmol) and NMM (66 μL, 0.60 μmol) for 18 h. The automated microwave-accelerated procedure was then used for the synthesis of peptide 48 on Fmoc-β-Asn(Otu)-Rink Amide resin (289 mg, 100 μmol). Quantities of HATU, DIPEA, piperidine and each Fmoc-amino acid were used as described by the automated protocols of the instrument and remained constant throughout this synthesis. The total amount of each coupling reagent and successive amino acid required, along with their reaction duration is summarised in the table below:—

TABLE Quantities of reagents and amino acids used in the synthesis of peptide 48 Total Mass (g) or Reaction Reagent volume (mL) Volume (mL) Time (min) 0.5M HATU in DMF 17.0 3.24 g — 2M DIPEA in NMP 9.0   3.1 mL — Fmoc-L-Asn(Trt)-OH 3.0 0.36 g 12 Fmoc-L-Agl-OH 6.0 0.41 g 12 Fmoc-L-Cys(Acm)-OH 3.0 0.25 g 12 Fmoc-L-Cys(^(t)Bu)—OH 3.0 0.24 g 12 Fmoc-L-Gln(Trt)-OH 3.0 0.37 g 12 Fmoc-L-Glu(O^(t)Bu)—OH 3.0 0.26 g 12 Fmoc-L-Ile-OH 3.0 0.21 g 12 Fmoc-L-Leu-OH 6.0 0.42 g 12 Fmoc-L-Ser(^(t)Bu)—OH 6.0 0.46 g 12 Fmoc-L-Thr(^(t)Bu)—OH 3.0 0.24 g 12 Fmoc-L-Tyr(^(t)Bu)—OH 6.0 0.55 g 12

After sequence completion, the resin-bound peptide was transferred into a fritted syringe and treated with an acetic anhydride solution (4 mL; DMF:acetic anhydride:NMM; 94:5:1) for 2 h. The resin was washed with DMF (4 mL; 3×1 min), DCM (4 mL; 3×1 min) and MeOH (4 mL; 3×1 min), then left to dry in vacuo for 1 h. Prior to treatment with MeOH, a small aliquot of the resin-bound peptide was removed and subjected to Fmoc-deprotection in the presence of 20% v/v piperidine in DMF (1 mL; 1×1 min, 2×10 min), and washed with DMF (1 mL; 5×1 min), DCM (1 mL; 3×1 min) and MeOH (1 mL; 3×1 min). The dried aliquot of Fmoc-deprotected resin-tethered peptide was subjected to TFA-mediated cleavage (General Section) and RP-HPLC and mass spectral analysis of the resultant solid supported formation of the desired peptide 48 in 85% purity. Mass spectrum (ESI⁺, MeCN:H₂O:HCOOH): m/z 986.7 [M+2H]²⁺, ½(C₈₇H₁₃₆N₂₀O₂₈S₂) requires 986.5. RP-HPLC (Agilent: Vydac C18 analytical column, 15→45% buffer B over 30 min): t_(R)=18.7 min.

des_(A1-5)-c[Δ⁴A6,11]-Dicarba-[A7]-Cys(^(t)Bu)-[A20]-Cys(Acm)-[A21]-β-Asn Human Insulin A-Chain 49

Resin-bound peptide 48 was subjected to the general microwave-accelerated RCM procedure outlined in the General Section under the following conditions: Resin-bound 48 (368 mg, 100 μmol), DCM (4.75 mL), 0.4 M LiCl in DMF (0.25 mL), 2^(nd) generation Grubbs' catalyst (17 mg, 20 μmol), 100 W pwave, 100° C., 2 h, 100% conversion into 49. Post metathesis, a small aliquot of resin-bound peptide was subjected to Fmoc-deprotection in the presence of 20% v/v piperidine in DMF (1 mL; 1×1 min, 2×10 min), then washed with DMF (1 mL; 5×1 min), DCM (1 mL; 3×1 min) and MeOH (1 mL; 3×1 min). The aliquot of Fmoc-deprotected resin-tethered peptide was subjected to TFA-mediated cleavage (General Section) for RP-HPLC and mass spectral analysis. This supported formation of the desired peptide as two isomers, 49(1) and 49(11), in a 75:25 ratio. 49(1): Mass spectrum (ESI⁺, MeCN:H₂O:HCOOH): m/z 972.5 [M+2H]²⁺, ½(C₈₅H₁₃₂N₂₀O₂₈S₂) requires 972.4. RP-HPLC (Agilent: Vydac C18 analytical column, 15→45% buffer B over 30 min): t_(R)=17.8 min. 49(11): Mass spectrum (ESI⁺, MeCN:H₂O:HCOOH): m/z 972.5 [M+2H]²⁺, ½(C₈₅H₁₃₂N₂₀O₂₈S₂) requires 972.4. RP-HPLC (Agilent: Vydac C18 analytical column, 15→45% buffer B over 30 min): t_(R)=18.0 min.

c[Δ⁴A6,11]-Dicarba[A7]-Cys(^(t)Bu)-[A20]-Cys(Acm)-[A21]-β-Asn Human Insulin A-Chain 50

The automated, microwave-accelerated procedure outlined in the General Section was used to attach the remaining 5 residues on resin-bound peptide 49 (352 mg, 100 μmol). Quantities of HATU, DIPEA, piperidine and each Fmoc-amino acid were used as described by the automated protocols of the instrument and remained constant throughout this synthesis. The total amount of each coupling reagent and successive amino acid required, along with their reaction duration is summarised in the table below:—

TABLE Quantities of reagents and amino acids used in the synthesis of peptide 50 Total Mass (g) or Reaction Reagent volume (mL) Volume (mL) Time (min) 0.5M HATU in DMF 6.0  1.14 g — 2M DIPEA in NMP 3.0    1.0 mL — Fmoc-L-Gly-OH 3.0 0.178 g 12 Fmoc-L-Gln(Trt)-OH 3.0 0.367 g 12 Fmoc-L-Glu(O^(t)Bu)—OH 3.0 0.256 g 12 Fmoc-L-Ile-OH 3.0 0.212 g 12 Fmoc-L-Val-OH 3.0 0.204 g 12

After sequence completion, the resin-bound peptide was transferred into a fritted syringe and subjected to Fmoc-deprotection in the presence of 20% v/v piperidine in DMF (7 mL; 1×1 min, 2×10 min). The resin was washed with DMF (7 mL; 5×1 min), DCM (7 mL; 3×1 min) and MeOH (7 mL; 3×1 min), then left to dry in vacuo for 1 h. The resin-bound peptide (397 mg) was subjected to TFA-mediated cleavage (General Section) and RP-HPLC and mass spectral analysis of the resultant pale brown solid (78 mg) supported formation of the desired peptide as two isomers, 50(I) and 50 (II), in a 75:25 ratio. 50 (I): Mass spectrum (ESI⁺, MeCN:H₂O:HCOOH): m/z 824.4 [M+3H]³⁺, ⅓(C₁₀₈H₁₇₁N₂₆O₃₆S₂) requires 824.1; 1235.6 [M+2H]²⁺, ½(C₁₀₈H₁₇₀N₂₆O₃₆S₂) requires 1235.6. RP-HPLC (Agilent: Vydac C18 analytical column, 15→45% buffer B over 30 min): t_(R)=18.4 min. 50 (II): Mass spectrum (ESI⁺, MeCN:H₂O:HCOOH): m/z 824.3 [M+3H]³⁺, ⅓(C₁₀₈H₁₇₁N₂₆O₃₆S₂) requires 824.1; 1235.7 [M+2H]²⁺, ½(C₁₀₈H₁₇₀N₂₆O₃₆S₂) requires 1235.6. RP-HPLC (Agilent: Vydac C18 analytical column, 15→45% buffer B over 30 min): t_(R)=19.4 min.

Ring Closing Alkyne Metathesis (RCAM) on Resin-Supported Peptides

Linear des_(A1-5/14-21)-[6,11]-Bgl Insulin A-Chain 17

Esterification of Fmoc-L-Leu-OH (106 mg, 300 μmol) on Wang resin (91 mg, 100 μmol) was performed according to the general protocol described in the General Section using DIC (47 μL, 300 μmol) and DMAP (3.7 mg, 30 μmol) for 2 h. The automated microwave-accelerated procedure was then used for the synthesis of peptide 17 on Fmoc-L-Leu-Wang resin (92 mg, 100 μmol). Quantities of HBTU, HOBt, DIPEA, piperidine and each Fmoc-amino acid were used as described by the automated protocols of the instrument and remained constant throughout this synthesis. The total amount of each coupling reagent and successive amino acid required, along with their reaction duration is summarised in the table below:—

TABLE Quantities of reagents and amino acids used in the synthesis of peptide 17 Total Mass (g) or Reaction Reagent volume (mL) Volume (mL) Time (min) 0.5M HBTU:HOBt in DMF 8 1.52:0.55 g — 2M DIPEA in NMP 4    1.4 mL — Fmoc-L-Cys(Trt)-OH 3 0.351 g 12 Fmoc-L-Bgl-OH 6 0.419 g 12 Fmoc-L-Ile-OH 3 0.212 g 12 Fmoc-L-Ser(^(t)Bu)—OH 6 0.460 g 12 Fmoc-L-Thr(^(t)Bu)—OH 3 0.238 g 12

After sequence completion, the resin-bound peptide was transferred into a fritted syringe and treated with an acetic anhydride solution (4 mL; DMF:acetic anhydride:NMM; 94:5:1) for 2 h. The resin was washed with DMF (4 mL; 3×1 min), DCM (4 mL; 3×1 min) and MeOH (4 mL; 3×1 min), then left to dry in vacuo for 1 h. Prior to treatment with MeOH, a small aliquot of the resin-bound peptide was removed and subjected to Fmoc-deprotection in the presence of 20% v/v piperidine in DMF (1 mL; 1×1 min, 2×10 min), and washed with DMF (1 mL; 5×1 min), DCM (1 mL; 3×1 min) and MeOH (1 mL; 3×1 min). The aliquot of Fmoc-deprotected resin-tethered peptide was subjected to TFA-mediated cleavage (General Section) and RP-HPLC and mass spectral analysis supported formation of the desired peptide 17. Mass spectrum (ESI⁺, MeCN:H₂O:HCOOH): m/z 841.3 [M+H]⁺, C₃₇H₆₁N₈O₁₂S requires 841.4. RP-HPLC (Vydac C18 analytical column, 0→30% buffer B over 30 min): t_(R)=28.9 min.

des_(A1-5/14-21)-c[Δ⁴A6,11]-Dao Insulin A-Chain 18

Resin-bound peptide 17 was dried via azeotropic distillation from THF and then subjected to the microwave-accelerated ROAM procedure outlined in the General Section under the following conditions: Resin-bound 17 (45 mg, 50 μmol), DCM (2 mL), Schrock's catalyst (4.7 mg, 10 μmol), 70° C., 3 h, 45% conversion into 18. Post metathesis, a small aliquot of resin-bound peptide was subjected to Fmoc-deprotection in the presence of 20% v/v piperidine in DMF (1 mL; 1×1 min, 2×10 min), then washed with DMF (1 mL; 5×1 min), DCM (1 mL; 3×1 min) and MeOH (1 mL; 3×1 min). The aliquot of Fmoc-deprotected resin-tethered peptide was subjected to TFA-mediated cleavage (General Section) for RP-HPLC and mass spectral analysis. This indicated a mixture of both cyclic and linear peptides, 18 and 17 respectively. Mass spectrum (ESI⁺, MeCN:H₂O:HCOOH): m/z 787.3 [M+H]⁺, (C₃₃H₅₅N₈O₁₂S) requires 787.4; 841.4. RP-HPLC (Vydac C18 analytical column, 0→30% buffer B over 30 min): t_(R(18))=21.0 min and t_(R(17))=28.9 min.

des_(A1-5/A14-21)-[A6,11]-Bgl-[A8]-Pro Insulin A-Chain 19

Esterification of Fmoc-L-Leu-OH (106 mg, 300 μmol) on Wang resin (91 mg, 100 μmol) was performed according to the general protocol described in the General Section using DIC (47 μL, 300 μmol) and DMAP (3.7 mg, 30 μmol) for 2 h. The automated microwave-accelerated procedure was then used for the synthesis of peptide 19 on Fmoc-L-Leu-Wang resin (92 mg, 100 μmol) for 2 h. Quantities of HBTU, HOBt, DIPEA, piperidine and each Fmoc-amino acid were used as described by the automated protocols of the instrument and remained constant throughout this synthesis. The total amount of each coupling reagent and successive amino acid required, along with their reaction duration is summarised in the Table below:—

TABLE Quantities of reagents and amino acids used in the synthesis of peptide 19 Total Mass (g) or Reaction Reagent volume (mL) Volume (mL) Time (min) 0.5M HBTU:HOBt in DMF 8 1.52 g:0.56 g — 2M DIPEA in NMP 4   1.4 mL — Fmoc-L-Cys(Trt)-OH 5 0.58 g 12 Fmoc-L-Bgl-OH 5 0.43 g 12 Fmoc-L-Ile-OH 5 0.34 g 12 Fmoc-L-Pro-OH 5 0.41 g 12 Fmoc-L-Ser(^(t)Bu)—OH 6 0.48 g 12

After sequence completion, the resin-bound peptide was transferred into a fritted syringe and treated with an acetic anhydride solution (4 mL; DMF:acetic anhydride:NMM; 94:5:1) for 2 h. The resin was washed with DMF (4 mL; 3×1 min), DCM (4 mL; 3×1 min) and MeOH (4 mL; 3×1 min), then left to dry in vacuo for 1 h. Prior to treatment with MeOH, a small aliquot of the resin-bound peptide was removed and subjected to Fmoc-deprotection in the presence of 20% v/v piperidine in DMF (1 mL; 1×1 min, 2×10 min), then washed with DMF (1 mL; 5×1 min), DCM (1 mL; 3×1 min) and MeOH (1 mL; 3×1 min). The aliquot of Fmoc-deprotected resin-tethered peptide was subjected to TFA-mediated cleavage (General Section) and RP-HPLC and mass spectral analysis supported formation of the desired peptide 19. Mass spectrum (ESI⁺, MeCN:H₂O:HCOOH): m/z 837.4 [M+H]⁺, C₃₈H₆₁N₈O₁₁S requires 837.4. RP-HPLC (Vydac C18 analytical column, 0→30% buffer B over 30 min): t_(R)=29.5 min.

des_(A1-5/A14-21)-c[Δ⁴A6,11]-Dao-[A8]-Pro Insulin A-Chain 20

Resin-bound peptide 19 was dried via azeotropic distillation from THF and then subjected to the general microwave-accelerated ROAM procedure outlined in the General Section under the following conditions: Resin-bound 19 (45 mg, 50 μmol), DCM (2 mL), Schrock's catalyst (4.7 mg, 10 μmol), 70° C., 3 h, 80% conversion into 20. Post metathesis, a small aliquot of resin-bound peptide was subjected to Fmoc-deprotection in the presence of 20% v/v piperidine in DMF (1 mL; 1×1 min, 2×10 min), and washed with DMF (1 mL; 5×1 min), DCM (1 mL; 3×1 min) and MeOH (1 mL; 3×1 min). The aliquot of Fmoc-deprotected resin-tethered peptide was subjected to TFA-mediated cleavage (General Section) for RP-HPLC and mass spectral analysis. This indicated a mixture of both cyclic and linear peptides, 20 and 19, respectively. Mass spectrum (ESI⁺, MeCN:H₂O:HCOOH): m/z 783.5 [M+H]⁺, C₃₄H₅₅N₈O₁₁S requires 783.4; 837.4. RP-HPLC (Vydac C18 analytical column, 0→30% buffer B over 30 min): t_(R(20))=23.2 min and t_(R(19))=29.5 min.

Latent Interchain Dicarba Bridges: An RCM Approach A Proline-Tethered A- and B-Chain Analogue of Human Insulin 140

The automated, microwave-accelerated procedure outlined was used for the synthesis of peptide 140 on Fmoc-Thr(^(t)Bu)-PEG-PS resin (625 mg, 100 μmol). Quantities of HATU, DIPEA, piperidine and each Fmoc-amino acid were used as described by the automated protocols of the instrument and remained constant throughout this synthesis. The total amount of each coupling reagent and successive amino acid required, along with their reaction duration is summarised in the table below:—

TABLE Quantities of reagents and amino acids used in the synthesis of peptide 140 Total Mass (g) or Reaction Reagent volume (mL) Volume (mL) Time (min) 0.5M HATU in DMF 18  3.43 g — 2M DIPEA in NMP 9    3.1 mL — Fmoc-L-Agl-OH 6.0 0.402 g 12 Fmoc-L-Asn(Trt)-OH 3.0 0.358 g 12 Fmoc-L-Cys(Trt)-OH 3.0 0.351 g 12 Fmoc-L-Gln(Trt)-OH 8.0 0.977 g 12 Fmoc-L-Glu(O^(t)Bu)—OH 3.0 0.255 g 12 Fmoc-L-Gly-OH 6.0 0.357 g 12 Fmoc-L-Ile-OH 3.0 0.212 g 12 Fmoc-L-Leu-OH 3.0 0.212 g 12 Fmoc-L-Phe-OH 3.0 0.232 g 12 Fmoc-L-Pro-OH 3.0 0.202 g 12 Fmoc-L-Val-OH 3.0 0.204 g 12

After sequence completion, the resin-bound peptide was transferred into a fritted syringe and treated with an acetic anhydride solution (7 mL; DMF:acetic anhydride:NMM; 94:5:1) for 2 h. The resin was washed with DMF (7 mL; 3×1 min), DCM (7 mL; 3×1 min) and MeOH (7 mL; 3×1 min), then left to dry in vacuo for 1 h. Prior to treatment with MeOH, a small aliquot of the resin-bound peptide was removed and subjected to Fmoc-deprotection in the presence of 20% v/v piperidine in DMF (1 mL; 1×1 min, 2×10 min), then washed with DMF (1 mL; 5×1 min), DCM (1 mL; 3×1 min) and MeOH (1 mL; 3×1 min). The aliquot of Fmoc-deprotected resin-tethered peptide was subjected to TFA-mediated cleavage (and mass spectral analysis supported formation of the desired peptide 140. Mass spectrum (ESI⁺, MeCN:H₂O:HCOOH): m/z 918.5 [M+2H]²⁺, ½(C₈₂H₁₂₈N₂₂O₂₄S) requires 918.5. RP-HPLC (Agilent: Vydac C18 analytical column, 0→30% buffer B over 5 min then 30→60% buffer B over 30 min): t_(R)=12.2 min.

RCM of the Proline-Tethered A- and B-Chain Analogue 140→141

Resin-bound peptide 140 was subjected to the general microwave-accelerated RCM procedure under the following conditions: Resin-bound 140 (496 mg, 50 μmol), DCM (6 mL), 2^(nd) generation Grubbs' catalyst (8.5 mg, 10 μmol), 0.4 M LiCl in DMF (0.2 mL), 100 W pwave, 100° C., 4 h, 78% conversion into 141. Post metathesis, a small aliquot of resin-bound peptide was subjected to Fmoc-deprotection in the presence of 20% v/v piperidine in DMF (1 mL; 1×1 min, 2×10 min), then washed with DMF (1 mL; 5×1 min), DCM (1 mL; 3×1 min) and MeOH (1 mL; 3×1 min). The aliquot of Fmoc-deprotected resin-tethered peptide was subjected to TFA-mediated cleavage for RP-HPLC and mass spectral analysis. This supported formation of the desired peptide as two isomers, 141(I) and 141(II), in a 1:1 ratio and a cyclic product 141 to linear starting material 140 ratio of 22:78. Mass spectrum (ESI⁺, MeCN:H₂O:HCOOH): m/z 904.6 [M+2H]²⁺, ½(C₈₀H₁₂₄N₂₂O₂₄S) requires 904.4; 918.7. RP-HPLC (Agilent: Vydac C18 analytical column, 0→30% buffer B over 5 min then 30→60% buffer B over 30 min): t_(R(141))=9.3 and 10.0 min and t_(R(140))=12.4 min.

A Truncated Proline-Tethered A- and B-Chain Analogue of Human Insulin 142

The automated, microwave-accelerated procedure was used for the synthesis of peptide 142 on Fmoc-Thr(^(t)Bu)-PEG-PS resin (625 mg, 100 μmol). Quantities of HATU, DIPEA, piperidine and each Fmoc-amino acid were used as described by the automated protocols of the instrument and remained constant throughout this synthesis. The total amount of each coupling reagent and successive amino acid required, along with their reaction duration is summarised in the table below:—

TABLE Quantities of reagents and amino acids used in the synthesis of peptide 142 Total Mass (g) or Reaction Reagent volume (mL) Volume (mL) Time (min) 0.5M HATU in DMF 12.0  2.29 g — 2M DIPEA in NMP 6.0    2.1 mL — Fmoc-L-Agl-OH 6.0 0.405 g 12 Fmoc-L-Cys(Trt)-OH 3.0 0.315 g 12 Fmoc-L-Gln(Trt)-OH 3.0 0.366 g 12 Fmoc-L-Glu(O^(t)Bu)—OH 3.0 0.255 g 12 Fmoc-L-Gly-OH 6.0 0.357 g 12 Fmoc-L-Ile-OH 3.0 0.212 g 12 Fmoc-L-Pro-OH 3.0 0.202 g 12 Fmoc-L-Val-OH 3.0 0.204 g 12

After sequence completion, the resin-bound peptide was transferred into a fritted syringe and treated with an acetic anhydride solution (7 mL; DMF:acetic anhydride:NMM; 94:5:1) for 2 h. The resin was washed with DMF (7 mL; 3×1 min), DCM (7 mL; 3×1 min) and MeOH (7 mL; 3×1 min), then left to dry in vacuo for 1 h. A small aliquot of the resin-bound peptide was removed and subjected to Fmoc-deprotection in the presence of 20% v/v piperidine in DMF (1 mL; 1×1 min, 2×10 min), then washed with DMF (1 mL; 5×1 min), DCM (1 mL; 3×1 min) and MeOH (1 mL; 3×1 min). The aliquot of Fmoc-deprotected resin-tethered peptide was exposed to a TFA cleavage solution and mass spectral analysis of the resultant grey solid supported the formation of the required peptide 142. Mass spectrum (ESI⁺, MeCN:H₂O:HCOOH): m/z 1097.3 [M+H]⁺, C₄₇H₇₇N₁₂O₁₆S requires 1097.3.

RCM of the Truncated Proline-Tethered A- and B-Chain Analogue of Human Insulin 142→203

Resin-bound peptide 142 was subjected to the general microwave-accelerated RCM procedure under the following conditions: Resin-bound 142 (416 mg, 50 μmol), DCM (6 mL), 2^(nd) generation Grubbs' catalyst (8.5 mg, 10 μmol), 0.4 M LiCl in DMF (0.2 mL), 100 W pwave, 100° C., 4 h, 100% conversion into 203. Post metathesis, a small aliquot of resin-bound peptide was subjected to Fmoc-deprotection in the presence of 20% v/v piperidine in DMF (1 mL; 1×1 min, 2×10 min), then washed with DMF (1 mL; 5×1 min), DCM (1 mL; 3×1 min) and MeOH (1 mL; 3×1 min). The aliquot of Fmoc-deprotected resin-tethered peptide was subjected to TFA-mediated cleavage and mass spectral analysis supported formation of the desired peptide 203. Mass spectrum (ESI⁺, MeCN:H₂O:HCOOH): m/z 1069.3 [M+H]⁺, C₄₅H₇₃N₁₂O₁₆S requires 1069.5. RP-HPLC (Agilent: Vydac C18 analytical column, 0→30% buffer B over 5 min then 30→60% buffer B over 30 min): t_(R)=12.5 min.

Addition of FVNQHL to Peptide 203→141

The automated, microwave-accelerated procedure was used to attach the remaining 6 residues on resin-bound peptide 203 (400 mg, 50 μmol). Quantities of HATU, DIPEA, piperidine and each Fmoc-amino acid were used as described by the automated protocols of the instrument and remained constant throughout this synthesis. The total amount of each coupling reagent and successive amino acid required, along with their reaction duration is summarised in the table below:—

TABLE Quantities of reagents and amino acids used in the synthesis of peptide 141 Total Mass (g) or Reaction Reagent volume (mL) Volume (mL) Time (min) 0.5M HATU in DMF 7.0  1.33 g — 2M DIPEA in NMP 4.0    1.4 mL — Fmoc-L-Asn(Trt)-OH 3.0 0.358 g 12 Fmoc-L-Gln(Trt)-OH 3.0 0.366 g 12 Fmoc-L-His(Trt)-OH 3.0 0.372 g 12 Fmoc-L-Leu-OH 3.0 0.212 g 12 Fmoc-L-Phe-OH 3.0 0.232 g 12 Fmoc-L-Val-OH 3.0 0.204 g 12

After sequence completion, the resin-bound peptide was transferred into a fritted syringe and washed with DMF (4 mL; 5×1 min), DCM (4 mL; 3×1 min) and MeOH (4 mL; 3×1 min), then left to dry in vacuo for 1 h. A small aliquot of resin was subjected to TFA-mediated cleavage and mass spectral analysis supported formation of the desired peptide 141. Mass spectrum (ESI⁺, MeCN:H₂O:HCOOH): m/z 904.4 [M+2H]²⁺, ½(C₈₀H₁₂₄N₂₂O₂₄S) requires 904.4. RP-HPLC (Agilent: Vydac C18 analytical column, 0→30% buffer B over 5 min then 30→60% buffer B over 30 min): t_(R)=9.3 and 9.9 min

5-Nitro-1,3-benzodioxane 204

A microwave vessel was loaded with meta-nitrophenol 144 (1.00 g, 7.19 mmol), formaldehyde solution (2 mL), and concentrated HCl (3 mL). The system was sealed and the reaction mixture then irradiated with microwaves (90 W) and stirred at 90° C. for 4 h. The resultant red solution was cooled to room temperature and diluted with water (15 mL) and EtOAc (15 mL). The phases were separated and the aqueous layer was further extracted with EtOAc (2×15 mL). The combined organic extract was washed with water (1×20 mL), dried (MgSO₄), filtered and concentrated under reduced pressure to afford a dark red oil (3.7 g). The crude product mixture was purified by column chromatography (SiO₂; light petroleum:EtOAc; 2:1) to afford the titled compound 204 as a colourless solid (0.73 g, 56%), m.p. 76-77° C. v_(max) (KBr): 3375 bs, 2968 m, 2927 m, 1611 m, 1581 w, 1530 s, 1487 w, 1457 m, 1392 w, 1349 s, 1286 m, 1212 m, 1172 w, 1151 w, 1078 w, 1037 w, 998 m, 943 w, 904 w, 837 m, 809 m, 791 m, 767 w, 740 m, 732 m cm⁻¹. ¹H NMR (400 MHz, CDCl₃): δ 5.21 (s, 2H, CCH₂O), 5.27 (s, 2H, OCH₂O), 7.18 (dd, J=8.3, 1.0 Hz, 1H, H2), 7.32 (t, J=8.3 Hz, 1H, H3), 7.81 (dd, J=8.2, 1.1 Hz, 1H, H4). ¹³C NMR (100 MHz, CDCl₃): δ 65.9 (CH₂OH), 90.9 (OCH₂O), 118.1 (C4), 118.7 (CCH₂), 123.3 (C2), 128.3 (C3), 145.3 (CNO₂), 154.3 (COH).

2-Hydroxymethyl-3-nitrophenol 205

A microwave vessel was loaded with 204 (0.26 g, 1.43 mmol) and 1 M HCl (6 mL). The vessel was sealed and the suspension was heated at reflux and monitored by TLC (SiO₂; light petroleum:EtOAc; 2:1). After 24 h, the product mixture was cooled to room temperature and diluted with EtOAc (15 mL). The phases were separated and the aqueous layer was further extracted with EtOAc (2×15 mL). The combined organic extract was washed with water (1×30 mL), dried (MgSO₄), filtered and concentrated under reduced pressure to afford a colourless solid (0.24 g). The crude product mixture was purified by column chromatography (SiO₂; light petroleum:EtOAc; 2:1) to give the desired product 205 as a colourless solid (0.22 g, 91%), m.p. 97-99° C. v_(max) (KBr): 3437 bs, 2968 m, 2929 m, 1643 m, 1612 m, 1531 s, 1488 w, 1456 m, 1393 w, 1349 s, 1285 s, 1212 m, 1172 w, 1151 w, 1078 w, 998 m, 943 w, 904 w, 837 m, 808 m, 791 m, 740 m, 732 m, 702 w, 637 m cm⁻¹. ¹H NMR (400 MHz, MeOD): δ 4.81 (bs, 2H, CH₂OH), 7.07 (m, 1H, H2), 7.25-7.31 (m, 2H, H3, 4). ¹³C NMR (100 MHz, MeOD): δ 54.7 (CH₂O), 114.5 (C4), 119.4 (C2), 120.8 (CCH₂), 128.6 (C3), 150.8 (CNO₂), 157.0 (COH).

2-Hydroxy-6-nitrobenzaldehyde 143

Manganese dioxide (1.13 g, 13.0 mmol) was added to a stirred solution of 205 (0.22 g, 1.30 mmol) in EtOAc (15 mL). The black suspension was stirred at room temperature and monitored by TLC (SiO₂; light petroleum:EtOAc; 2:1). After 16 h, the reaction mixture was filtered though a celite plug and concentrated in vacuo to give a brown solid. The crude product mixture was purified by column chromatography (SiO₂; light petroleum:EtOAc; 2:1) to give the titled compound as a yellow solid (0.17 g, 76%), m.p. 52-53° C. (lit. 53-54° C.). V_(max) (KBr): 3424 bs, 3107 m, 2928 m, 1656 s, 1560 w, 1529 s, 1450 m, 1355 s, 1283 s, 1195 m, 1176 m, 1066 w, 969 m, 842 m, 815 m, 785 s, 735 s, 695 m cm⁻¹. ¹H NMR (400 MHz, CDCl₃): δ 7.26 (d, J=8.4 Hz, 1H, H2), 7.52 (dd, J=7.8, 0.9 Hz, 1H, H3), 7.61 (t, J=8.1 Hz, 1H, H4), 10.27 (s, 1H, CHO), 12.03 (s, 1H, OH). ¹³C NMR (100 MHz, MeOD): δ 112.4 (C4), 116.1 (CCHO), 124.2 (C2), 136.0 (C3), 151.3 (CNO₂), 163.3 (COH), 193.9 (CHO).

des_(A10-21)-[A7]-Agl Human Insulin A-Chain 145

The automated, microwave-accelerated procedure was used for the synthesis of peptide 145 on rink amide resin (192 mg, 100 μmol). Quantities of HATU, DIPEA, piperidine and each Fmoc-amino acid were used as described by the automated protocols of the instrument and remained constant throughout this synthesis. The total amount of each coupling reagent and successive amino acid required, along with their reaction duration is summarised in the table below:—

TABLE Quantities of reagents and amino acids used in the synthesis of peptide 145 Total Mass (g) or Reaction Reagent volume (mL) Volume (mL) Time (min) 0.5M HATU in DMF 10.0  1.91 g — 2M DIPEA in NMP 5.0    1.7 mL — Fmoc-L-Agl-OH 3.0 0.202 g 12 Fmoc-L-Cys(Trt)-OH 3.0 0.315 g 12 Fmoc-L-Gln(Trt)-OH 3.0 0.366 g 12 Fmoc-L-Glu(O^(t)Bu)—OH 3.0 0.255 g 12 Fmoc-L-Gly-OH 3.0 0.178 g 12 Fmoc-L-Ile-OH 3.0 0.212 g 12 Fmoc-L-Ser(^(t)Bu)—OH 3.0 0.230 g 12 Fmoc-L-Thr(^(t)Bu)—OH 3.0 0.238 g 12 Fmoc-L-Val-OH 3.0 0.204 g 12

After sequence completion, the resin-bound peptide was transferred into a fritted syringe and washed with DMF (7 mL; 3×1 min), DCM (7 mL; 3×1 min) and MeOH (7 mL; 3×1 min), then left to dry in vacuo for 1 h. A small aliquot of Fmoc-deprotected resin-tethered peptide was subjected TFA-mediated cleavage for RP-HPLC and mass spectral analysis. This supported formation of the desired peptide 145 in >95% purity. Mass spectrum (ESI⁺, MeCN:H₂O:HCOOH): m/z 932.2 [M+H]⁺, C₃₈H₆₆N₁₁O₁₄S requires 932.5. RP-HPLC (Agilent: Vydac C18 analytical column, 15→45% buffer B over 30 min): t_(R)=21.3 min.

Incorporation of HnB and Boc-Protection of A-Chain Analogue 145→146

Resin-bound peptide 145 was swollen with DCM (4 mL; 3×1 min, 1×60 min) and DMF (4 mL; 3×1 min, 1×30 min) then subjected to reductive amination: 2-Hydroxy-6-nitrobenzaldehyde 143 (33 mg, 200 μmol) in MeOH:DMF (2 mL, 1:1) was added to the resin-tethered peptide 145 (310 mg, 100 μmol) and the reaction allowed to shake gently for 30 min. The resin was filtered and a second portion of aldehyde 143 (33 mg, 200 μmol) in MeOH:DMF (2 mL, 1:1) was added. After 2 h, the resin was again filtered, washed with MeOH:DMF (1:1; 4 mL, 3×1 min) and treated with a solution of sodium borohydride (39 mg, 1.0 mmol) in MeOH:DMF (5 mL, 1:3) for 30 min. After this reaction duration, the resin was filtered, washed with MeOH:DMF (1:3; 4 mL, 3×1 min), DMF (4 mL, 3×1 min) and MeOH:DCM (1:1; 4 mL, 3×1 min), then left to dry in vacuo for 1 h. A small aliquot of resin was subjected TFA-mediated cleavage and RP-HPLC and mass spectral analysis confirmed 90% conversion to the required peptide 146. Mass spectrum (ESI⁺, MeCN:H₂O:HCOOH): m/z 933.0; 1083.2 [M+H]⁺, C₄₅H₇₁N₁₂O₁₇S requires 1083.5. RP-HPLC (Agilent: Vydac C18 analytical column, 15→45% buffer B over 30 min): t_(R(146))=9.3 min and t_(R(145))=21.4 min.

The resin-bound peptide was then re-swollen with DCM (4 mL; 3×1 min, 1×60 min) and DMF (4 mL; 3×1 min, 1×30 min) and the secondary amine then Boc-protected: Di-tert-butyldicarbonate (65.5 mg, 300 μmol) in dry DCM was added to the resin-tethered peptide and shaken gently at room temperature. After 16 h, a chloranil test was negative for the presence of secondary amines and hence the peptide was washed with DMF (7 mL; 3×1 min), DCM (7 mL; 3×1 min) and MeOH (7 mL; 3×1 min), then left to dry in vacuo for 1 h.

Esterification of A-Chain Analogue 146→147

DMAP (1.2 mg, 10 μmol) in dry DCM (0.5 mL) was added dropwise to a stirred suspension of resin-bound peptide 146 (280 mg, 100 μmol), Fmoc-L-Gly-OH (89.2 mg, 300 μmol) and EDCI.HCl (57.4 mg, 300 μmol) in dry DCM:DMF (4 mL; 1:1). After 14 h, the resin was filtered and washed with DMF (7 mL; 3×1 min), DCM (7 mL; 3×1 min) and MeOH (7 mL; 3×1 min), then left to dry in vacuo for 1 h. A small aliquot of resin was subjected TFA-mediated cleavage and mass spectral analysis supported formation of the required peptide 147. Mass spectrum (ESI⁺, MeCN:H₂O:HCOOH): m/z 681.6 [M+2H]²⁺, ½(C₆₂H₈₅N₁₃O₂₀S) requires 681.8; 1083.3; 1212.4; 1362.3 [M+H]⁺, C₆₂H₈₄N₁₃O₂₀S requires 1362.6. RP-HPLC (Agilent: Vydac C18 analytical column, 15→45% buffer B over 30 min): t_(R(147))=29.5 min.

Addition of Fmoc-L-Agl-OH to A-Chain Analogue 147→206

A solution of Fmoc-L-Agl-OH (101 mg, 300 μmol), HATU (114 mg, 300 μmol) and NMM (66 μL, 0.60 μmol) in dry DMF (3 mL) was added to the resin-tethered peptide 147 (280 mg, 100 pmol) and shaken gently at room temperature. After 3 h, the resin was filtered and washed with DMF (7 mL; 3×1 min), DCM (7 mL; 3×1 min) and MeOH (7 mL; 3×1 min), then left to dry in vacuo for 1 h. A small aliquot of resin-bound peptide was subjected TFA-mediated cleavage and mass spectral analysis only gave a molecular ion for the hydrolysed peptide 146. The ester linkage was prone to acid-hydrolysis and rapidly decomposed into peptide 146 during cleavage. Mass spectrum (ESI⁺, MeCN:H₂O:HCOOH): m/z 702.0; 1083.1; 1235.3; 1252.4; 1404.2.

RCM of the HnB Tethered A- and B-Chain Analogue 206→148

Resin-bound peptide 206 was subjected to the general microwave-accelerated RCM procedure under the following conditions: Resin-bound 206 (280 mg, 100 μmol), DCM (3 mL), 0.4 M LiCl in DMF (0.1 mL), 2^(nd) generation Grubbs' catalyst (17 mg, 20 μmol), 100 W pwave, 100° C., 4 h. Post metathesis, a small aliquot of resin-bound peptide was subjected to TFA-mediated cleavage and mass spectral analysis supported formation of the desired peptide 148. Hydrolysed peptide 146 was also observed. Mass spectrum (ESI⁺, MeCN:H₂O:HCOOH): m/z 716.1 [M+2H]²⁺, C₆₅H₈₈N₁₄O₂₁S requires 716.3; 1083.2; 1159.2; 1224.3, 1431.2 [M+H]⁺, C₆₅H₈₇N₁₄O₂₀S requires 1431.6. RP-HPLC (Agilent: Vydac C18 analytical column, 15→45% buffer B over 30 min): t_(R(148))=22.2 min.

Preparation of c[Das-API]-SL-c[Das-API]-SL-c[Das-API]-SLG 68 Agl-API-Agl-SLG 60

The automated, microwave-accelerated procedure outlined in the General Section was used for the synthesis of peptide 60 on Fmoc-Gly-Wang resin (377 mg, 200 μmol). Quantities of HBTU, HOBt DIPEA, piperidine and each Fmoc-amino acid were used as described by the automated protocols of the instrument and remained constant throughout this synthesis. The total amount of each coupling reagent and successive amino acid required, along with their reaction duration is summarised in the table below:—

TABLE Quantities of reagents and amino acids used in the synthesis of peptide 60 Total Mass (g) or Reaction Reagent volume (mL) Volume (mL) Time (min) 0.5M HBTU:HOBt in DMF 16 3.03 g:1.08 g — 2M DIPEA in NMP 8    2.8 mL — Fmoc-L-Agl-OH 10 0.675 g 12 Fmoc-L-Ala-OH 5 0.311 g 12 Fmoc-L-Ile-OH 5 0.353 g 12 Fmoc-L-Leu-OH 5 0.353 g 12 Fmoc-L-Pro-OH 5 0.337 g 12 Fmoc-L-Ser(^(t)Bu)—OH 5 0.383 g 12

After sequence completion, the resin-bound peptide was transferred into a fritted syringe and treated with an acetic anhydride solution (7 mL; DMF:acetic anhydride:NMM; 94:5:1) for 2 h. The resin was then washed with DMF (7 mL; 3×1 min), DCM (7 mL; 3×1 min) and MeOH (7 mL; 3×1 min), then left to dry in vacuo for 1 h. Prior to treatment with MeOH, a small aliquot of the resin-bound peptide was removed and subjected to Fmoc-deprotection in the presence of 20% v/v piperidine in DMF (7 mL; 1×1 min, 2×10 min), then washed with DMF (7 mL; 5×1 min), DCM (7 mL; 3×1 min) and MeOH (7 mL; 3×1 min). The dried aliquot of Fmoc-deprotected resin-tethered peptide was subjected to TFA-mediated cleavage (General Section) for RP-HPLC and mass spectral analysis. This supported formation of the desired peptide 60 in 92% purity. Mass spectrum (ESI⁺, MeCN:H₂O:HCOOH): m/z 751.3 [M+H]⁺, C₃₅H₅₉N₈O₁₀ requires 751.4. RP-HPLC (Vydac C18 analytical column, 0→30% buffer B over 30 min): t_(R)=23.6 min.

c[Δ⁴Das-API]SLG 61

Resin-bound Fmoc-protected peptide 60 was subjected to the general microwave-accelerated RCM procedure outlined in the General Section under the following conditions: Resin-bound 60 (527 mg, 200 μmol), DCM (4.75 mL), 0.4 M LiCl in DMF (0.25 mL), 2^(nd) generation Grubbs' catalyst (34 mg, 40 μmol), 100 W pwave, 80° C., 4 h, 100% conversion into 61. Post metathesis, a small aliquot of resin-bound peptide was subjected to Fmoc-deprotection in the presence of 20% v/v piperidine in DMF (7 mL; 1×1 min, 2×10 min), then washed with DMF (7 mL; 5×1 min), DCM (7 mL; 3×1 min) and MeOH (7 mL; 3×1 min). The dried aliquot of Fmoc-deprotected resin-tethered peptide was subjected to TFA-mediated cleavage (General Section) and RP-HPLC and mass spectral analysis of the resultant isolated solid supported formation of the required unsaturated carbocycle 61. Mass spectrum (ESI⁺, MeCN:H₂O:HCOOH): m/z 636.2; 723.2 [M+H]⁺, C₃₃H₅₅N₈O₁₀ requires 723.4. RP-HPLC (Vydac C18 analytical column, 0→30% buffer B over 30 min): t_(R)=23.7 min (broad).

c[Das-API]-SLG 62

Resin-bound peptide 61 was subjected to the microwave-accelerated hydrogenation procedure described previously under the following conditions: Resin-bound 61 (514 mg, 200 μmol), DCM (4.5 mL), MeOH (0.5 mL), Wilkinson's catalyst, H₂ (80 psi), 100 W pwave, 80° C., 4 h, 100% conversion into 62. Following hydrogenation, a small aliquot of resin-bound peptide was subjected to Fmoc-deprotection in the presence of 20% v/v piperidine in DMF (7 mL; 1×1 min, 2×10 min), then washed with DMF (7 mL; 5×1 min), DCM (7 mL; 3×1 min) and MeOH (7 mL; 3×1 min). The dried aliquot of Fmoc-deprotected resin-tethered peptide was subjected to TFA-mediated cleavage (General Section) and RP-HPLC and mass spectral analysis of the resultant isolated solid supported formation of the required saturated carbocycle 62. Mass spectrum (ESI⁺, MeCN:H₂O:HCOOH): m/z 638.2; 725.3 [M+H]⁺, C₃₃H₅₇N₈O₁₀ requires 725.4. RP-HPLC (Vydac C18 analytical column, 0→30% buffer B over 30 min): t_(R)=24.7 min.

AgI-API-AgI-SL-c[Das-API]-SLG 63

Synthesis of the extended sequence 63 was performed according to the microwave-accelerated SPPS procedure described previously on Fmoc-cyclic peptide-Wang resin 62 (490 mg, 200 μmol). Quantities of HBTU, HOBt DIPEA, piperidine and each Fmoc-amino acid were used as described by the automated protocols of the instrument and remained constant throughout this synthesis. The total amount of each coupling reagent and successive amino acid required, along with their reaction duration is summarised in the table below:—

TABLE Quantities of reagents and amino acids used in the synthesis of peptide 63. Total Mass (g) or Reaction Reagent volume (mL) Volume (mL) Time (min) 0.5M HBTU:HOBt in DMF 16 3.03 g:1.08 g — 2M DIPEA in NMP 8    2.8 mL — Fmoc-L-Agl-OH 10 0.675 g 12 Fmoc-L-Ala-OH 5 0.311 g 12 Fmoc-L-Ile-OH 5 0.353 g 12 Fmoc-L-Leu-OH 5 0.353 g 12 Fmoc-L-Pro-OH 5 0.337 g 12 Fmoc-L-Ser(^(t)Bu)—OH 5 0.383 g 12

After sequence completion, the resin-bound peptide was transferred into a fritted syringe and treated with an acetic anhydride solution (7 mL; DMF:acetic anhydride:NMM; 94:5:1) for 2 h. The resin was then washed with DMF (7 mL; 3×1 min), DCM (7 mL; 3×1 min) and MeOH (7 mL; 3×1 min), then left to dry in vacuo for 1 h. Prior to treatment with MeOH, a small aliquot of the resin-bound peptide was removed and subjected to Fmoc-deprotection in the presence of 20% v/v piperidine in DMF (7 mL; 1×1 min, 2×10 min), then washed with DMF (7 mL; 5×1 min), DCM (7 mL; 3×1 min) and MeOH (7 mL; 3×1 min). The dried aliquot of Fmoc-deprotected resin-tethered peptide was subjected to TFA-mediated cleavage (General Section) for RP-HPLC and mass spectral analysis. This supported formation of the desired peptide 63 in 89% purity. Mass spectrum (ESI⁺, MeCN:H₂O:HCOOH): m/z 700.9 [M+2H]²⁺, ½(C₆₆H₁₁₁N₁₅O₁₈) requires 700.9; 1400.8 [M+H]⁺, C₆₆H₁₁₀N₁₅O₁₈ requires 1400.8. RP-HPLC (Vydac C18 analytical column, 15→45% buffer B over 30 min): t_(R)=19.1 min.

c[Δ⁴Das-API]-c[Das-API]-SLG 64

Resin-bound Fmoc-protected peptide 63 was subjected to the general microwave-accelerated RCM procedure outlined in the General Section under the following conditions: Resin-bound 63 (579 mg, 200 μmol), DCM (4.75 mL), 0.4 M LiCl in DMF (0.25 mL), 2^(nd) generation Grubbs' catalyst (34 mg, 40 μmol), 100 W pwave, 80° C., 4 h, 100% conversion into 64. Post metathesis, a small aliquot of resin-bound peptide was subjected to Fmoc-deprotection in the presence of 20% v/v piperidine in DMF (7 mL; 1×1 min, 2×10 min), then washed with DMF (7 mL; 5×1 min), DCM (7 mL; 3×1 min) and MeOH (7 mL; 3×1 min). The dried aliquot of Fmoc-deprotected resin-tethered peptide was subjected to TFA-mediated cleavage (General Section) and RP-HPLC and mass spectral analysis of the resultant isolated solid supported formation of the required unsaturated bicyclic peptide as two isomers, 64(I) and 64(II), in a 4:6 ratio. 64(I): Mass spectrum (ESI⁺, MeCN:H₂O:HCOOH): m/z 686.9 [M+2H]²⁺, ½(C₆₄H₁₀₇N₁₅O₁₈) requires 686.9; 1372.6 [M+H]⁺, C₆₄H₁₀₆N₁₅O₁₈ requires 1372.8. RP-HPLC (Vydac C18 analytical column, 15→45% buffer B over 30 min): t_(R)=19.1 min (broad). 64(11): Mass spectrum (ESI⁺, MeCN:H₂O:HCOOH): m/z 643.2; 686.9 [M+2H]²⁺, ½(C₆₄H₁₀₇N₁₅O₁₈) requires 686.9; 1372.6 [M+H]⁺, C₆₄H₁₀₆N₁₅O₁₈ requires 1372.8. RP-HPLC (Vydac C18 analytical column, 15→45% buffer B over 30 min): t_(R)=19.5 min (broad).

c[Das-API]SL-c[Das-API]SLG 65

Resin-bound peptide 64 was subjected to the microwave-accelerated hydrogenation procedure described previously under the following conditions: Resin-bound 64 (564 mg, 200 pmol), DCM (4.5 mL), MeOH (0.5 mL), Wilkinson's catalyst, H₂ (80 psi), 100 W pwave, 80° C., 4 h, 100% conversion into 65. Following hydrogenation, a small aliquot of resin-bound peptide was subjected to Fmoc-deprotection in the presence of 20% v/v piperidine in DMF (7 mL; 1×1 min, 2×10 min), then washed with DMF (7 mL; 5×1 min), DCM (7 mL; 3×1 min) and MeOH (7 mL; 3×1 min). The dried aliquot of Fmoc-deprotected resin-tethered peptide was subjected to TFA-mediated cleavage (General Section) and RP-HPLC and mass spectral analysis of the resultant isolated solid supported formation of the required saturated bicylic peptide 65. Mass spectrum (ESI⁺, MeCN:H₂O:HCOOH): m/z 688.0 [M+2H]²⁺, ½(C₆₄H₁₀₉N₁₅O₁₈) requires 687.9; 1374.9 [M+H]⁺, C₆₄H₁₀₈N₁₅O₁₈ requires 1374.8. RP-HPLC (Vydac C18 analytical column, 15→45% buffer B over 30 min): t_(R)=20.3 min.

AgI-API-AgI-SL-c[Das-API]-SL-c[Das-API]-SLG 66

Synthesis of the extended sequence 66 was performed according to the microwave-accelerated SPPS procedure described previously on Fmoc-bicyclic peptide-Wang resin 65 (558 mg, 200 μmol). Quantities of HBTU, HOBt DIPEA, piperidine and each Fmoc-amino acid were used as described by the automated protocols of the instrument and remained constant throughout this synthesis. The total amount of each coupling reagent and successive amino acid required, along with their reaction duration is summarised in the table below:—

TABLE Quantities of reagents and amino acids used in the synthesis of peptide 66 Total Mass (g) or Reaction Reagent volume (mL) Volume (mL) Time (min) 0.5M HBTU:HOBt in DMF 16 3.03 g:1.08 g — 2M DIPEA in NMP 8    2.8 mL — Fmoc-L-Agl-OH 10 0.675 g 12 Fmoc-L-Ala-OH 5 0.311 g 12 Fmoc-L-Ile-OH 5 0.353 g 12 Fmoc-L-Leu-OH 5 0.353 g 12 Fmoc-L-Pro-OH 5 0.337 g 12 Fmoc-L-Ser(^(t)Bu)—OH 5 0.383 g 12

After sequence completion, the resin-bound peptide was transferred into a fritted syringe and treated with an acetic anhydride solution (7 mL; DMF:acetic anhydride:NMM; 94:5:1) for 2 h. The resin was then washed with DMF (7 mL; 3×1 min), DCM (7 mL; 3×1 min) and MeOH (7 mL; 3×1 min), then left to dry in vacuo for 1 h. Prior to treatment with MeOH, a small aliquot of the resin-bound peptide was removed and subjected to Fmoc-deprotection in the presence of 20% v/v piperidine in DMF (7 mL; 1×1 min, 2×10 min), then washed with DMF (7 mL; 5×1 min), DCM (7 mL; 3×1 min) and MeOH (7 mL; 3×1 min). The dried aliquot of Fmoc-deprotected resin-tethered peptide was subjected to TFA-mediated cleavage (General Section) for RP-HPLC and mass spectral analysis. This supported formation of the required sidechain-deprotected peptide 66 in 71% purity. Mass spectrum (ESI⁺, MeCN:H₂O:HCOOH): m/z 684.3 [M+3H]³⁺, ⅓(C₉₇H₁₆₃N₂₂O₂₆) requires 684.1; 1025.7 [M+2H]²⁺, ½(C₉₇H₁₆₂N₂₂O₂₆) requires 1025.6. RP-HPLC (Vydac C18 analytical column, 15→45% buffer B over 30 min): t_(R)=25.5 min.

c[Δ⁴Das-API]SL-c[Das-API]SL-c[Das-API]-SLG 67

Resin-bound Fmoc-protected peptide 66 was subjected to the general microwave-accelerated RCM procedure outlined in the General Section under the following conditions: Resin-bound 66 (617 mg, 200 μmol), DCM (4.75 mL), 0.4 M LiCl in DMF (0.25 mL), 2^(nd) generation Grubbs' catalyst (34 mg, 40 μmol), 100 W pwave, 80° C., 4 h, 95% conversion into 67. Post metathesis, a small aliquot of resin-bound peptide was subjected to Fmoc-deprotection in the presence of 20% v/v piperidine in DMF (7 mL; 1×1 min, 2×10 min), then washed with DMF (7 mL; 5×1 min), DCM (7 mL; 3×1 min) and MeOH (7 mL; 3×1 min). The dried aliquot of Fmoc-deprotected resin-tethered peptide was subjected to TFA-mediated cleavage (General Section) and RP-HPLC and mass spectral analysis of the resultant isolated solid supported formation of the required unsaturated tricyclic peptide as two isomers, 67(I) and 67II), in a 4:6 ratio. 67(I): Mass spectrum (ESI⁺, MeCN:H₂O:HCOOH): m/z 675.0 [M+3H]³⁺, ⅓(C₉₅H₁₅₉N₂₂O₂₆) requires 674.7; 1011.7 [M+2H]²⁺, ½(C₉₅H₁₅₈N₂₂O₂₆) requires 1011.6. RP-HPLC (Vydac C18 analytical column, 15→45% buffer B over 30 min): t_(R)=25.6 min. 67(11): Mass spectrum (ESI⁺, MeCN:H₂O:HCOOH): m/z 675.0 [M+3H]³⁺, ⅓(C₉₅H₁₅₉N₂₂O₂₆) requires 674.7; 1011.7 [M+2H]²⁺, ½(C₉₅H₁₅₈N₂₂O₂₆) requires 1011.6. RP-HPLC (Vydac C18 analytical column, 15→45% buffer B over 30 min): t_(R)=27.1 min.

c[Das-API]-SL-c[Das-API]-SL-c[Das-API]-SLG 68

Resin-bound peptide 67 was subjected to the microwave-accelerated hydrogenation procedure described previously under the following conditions: Resin-bound 67 (615 mg, 200 pmol), DCM (4.5 mL), MeOH (0.5 mL), Wilkinson's catalyst, H₂ (80 psi), 100 W pwave, 80° C., 4 h, 100% conversion into 68. Following hydrogenation, a small aliquot of resin-bound peptide was subjected to Fmoc-deprotection in the presence of 20% v/v piperidine in DMF (7 mL; 1×1 min, 2×10 min), then washed with DMF (7 mL; 5×1 min), DCM (7 mL; 3×1 min) and MeOH (7 mL; 3×1 min). The dried aliquot of Fmoc-deprotected resin-tethered peptide was subjected to TFA-mediated cleavage (General Section) and RP-HPLC and mass spectral analysis of the resultant isolated solid supported formation of the required saturated tricyclic peptide 68. Mass spectrum (ESL MeCN:H₂O:HCOOH): m/z 675.5 [M+3H]³⁺, ⅓(C₉₅H₁₆₁N₂₂O₂₆) requires 675.4; 1012.7 [M+2H]²⁺, ½(C₉₅H₁₆₀N₂₂O₂₆) requires 1012.6. RP-HPLC (Vydac C18 analytical column, 15→45% buffer B over 30 min): t_(R)=26.1 min.

Following global Fmoc-deprotection and TFA-mediated cleavage of the remaining peptide 68 from the resin (564 mg), the resultant pale brown solid (423 mg) was purified by RP-HPLC (Agilent: Vydac C18 preparative column, 20→45% buffer B over 30 min, t_(R)=27.0 min). Selected fractions were combined and lyophilised to give the desired peptide 68 as a colourless solid (12.6 mg, 3%) in >99% purity. Mass spectrum (ESI⁺, MeCN:H₂O:HCOOH): m/z 1012.6 [M+2H]²⁺, ½(C₉₅H₁₆₀N₂₂O₂₆) requires 1012.6. RP-HPLC (Vydac C18 analytical column, 20→45% buffer B over 30 min): t_(R)=24.2 min.

Preparation of c[Das-API]SR-c[Das-API]-SR-c[Das-API]-SRG 69 AgI-API-AgI-SRG 70

The automated, microwave-accelerated procedure outlined in the General Section was used for the synthesis of peptide 70 on Fmoc-Gly-Wang resin (169 mg, 100 μmol). Quantities of HBTU, HOBt DIPEA, piperidine and each Fmoc-amino acid were used as described by the automated protocols of the instrument and remained constant throughout this synthesis. The total amount of each coupling reagent and successive amino acid required, along with their reaction duration is summarised in the table below:—

TABLE Quantities of reagents and amino acids used in the synthesis of peptide 70 Total Mass (g) or Reaction Reagent volume (mL) Volume (mL) Time (min) 0.5M HBTU:HOBt in DMF 16 3.03 g:1.08 g — 2M DIPEA in NMP 8    2.8 mL — Fmoc-L-Agl-OH 10 0.675 g 12 Fmoc-L-Ala-OH 5 0.311 g 12 Fmoc-L-Arg(Pbf)-OH 5 0.353 g 12 Fmoc-L-Ile-OH 5 0.353 g 12 Fmoc-L-Pro-OH 5 0.337 g 12 Fmoc-L-Ser(^(t)Bu)—OH 5 0.383 g 12

After sequence completion, the resin-bound peptide was transferred into a fritted syringe and treated with an acetic anhydride solution (7 mL; DMF:acetic anhydride:NMM; 94:5:1) for 2 h. The resin was then washed with DMF (7 mL; 3×1 min), DCM (7 mL; 3×1 min) and MeOH (7 mL; 3×1 min), then left to dry in vacuo for 1 h. Prior to treatment with MeOH, a small aliquot of the resin-bound peptide was removed and subjected to Fmoc-deprotection in the presence of 20% v/v piperidine in DMF (7 mL; 1×1 min, 2×10 min), then washed with DMF (7 mL; 5×1 min), DCM (7 mL; 3×1 min) and MeOH (7 mL; 3×1 min). The dried aliquot of Fmoc-deprotected resin-tethered peptide was subjected to TFA-mediated cleavage (General Section) for RP-HPLC and mass spectral analysis. This supported formation of the desired peptide 70 in 92% purity. Mass spectrum (ESI⁺, MeCN:H₂O:HCOOH): m/z 397.8 [M+2H]²⁺, C₃₅H₆₁N₁₁O₁₀ requires 397.7; 794.6 [M+H]⁺, C₃₅H₆₀N₁₁O₁₀ requires 794.5. RP-HPLC (Vydac C18 analytical column, 0→30% buffer B over 30 min): t_(R)=15.9 min.

c[Δ⁴Das-API]SRG 71

Resin-bound Fmoc-protected peptide 70 was subjected to the general microwave-accelerated RCM procedure outlined in the General Section under the following conditions: Resin-bound 70 (338 mg, 100 μmol), DCM (4.75 mL), 0.4 M LiCl in DMF (0.25 mL), 2^(nd) generation Grubbs' catalyst (17 mg, 20 μmol), 100 W pwave, 80° C., 4 h, 100% conversion into 71. Post metathesis, a small aliquot of resin-bound peptide was subjected to Fmoc-deprotection in the presence of 20% v/v piperidine in DMF (7 mL; 1×1 min, 2×10 min), then washed with DMF (7 mL; 5×1 min), DCM (7 mL; 3×1 min) and MeOH (7 mL; 3×1 min). The aliquot of Fmoc-deprotected resin-tethered peptide was subjected to TFA-mediated cleavage (General Section) and RP-HPLC and mass spectral analysis of the resultant isolated solid supported formation of the required unsaturated carbocycle as two isomers, 71(I) and 71(II), in a 7:3 ratio. 71(I): Mass spectrum (ESI⁺, MeCN:H₂O:HCOOH): m/z 383.7 [M+2H]²⁺, C₃₃H₅₇N₁₁O₁₀ requires 383.7; 766.5 [M+H]⁺, C₃₃H₅₆N₁₁O₁₀ requires 766.4. RP-HPLC (Vydac C18 analytical column, 0→30% buffer B over 30 min): t_(R)=15.3 min (broad). 71(II): Mass spectrum (ESI⁺, MeCN:H₂O:HCOOH): m/z 383.7 [M+2H]²⁺, C₃₃H₅₇N₁₁O₁₀ requires 383.7; 766.5 [M+H]⁺, C₃₃H₅₆N₁₁O₁₀ requires 766.4. RP-HPLC (Vydac C18 analytical column, 0→30% buffer B over 30 min): t_(R)=15.5 min (broad).

c[Das-API]-SRG 72

Resin-bound peptide 71 was subjected to the microwave-accelerated hydrogenation procedure described previously under the following conditions: Resin-bound 71 (321 mg, 100 μmol), DCM (4.5 mL), MeOH (0.5 mL), Wilkinson's catalyst, H₂ (80 psi), 100 W pwave, 80° C., 4 h, 100% conversion into 72. Following hydrogenation, a small aliquot of resin-bound peptide was subjected to Fmoc-deprotection in the presence of 20% v/v piperidine in DMF (7 mL; 1×1 min, 2×10 min), then washed with DMF (7 mL; 5×1 min), DCM (7 mL; 3×1 min) and MeOH (7 mL; 3×1 min). The aliquot of Fmoc-deprotected resin-tethered peptide was subjected to TFA-mediated cleavage (General Section) and RP-HPLC and mass spectral analysis of the resultant isolated solid supported formation of the required saturated carbocycle 72. Mass spectrum (ESI⁺, MeCN:H₂O:HCOOH): m/z 384.8 [M+2H]²⁺, C₃₃H₅₉N₁₁O₁₀ requires 384.7; 768.6 [M+H]⁺, C₃₃H₅₈N₁₁O₁₀ requires 768.4. RP-HPLC (Vydac C18 analytical column, 0→30% buffer B over 30 min): t_(R)=15.3 min.

AgI-API-AgI-SR-c[Das-API]-SRG 73

Synthesis of the extended sequence 73 was performed according to the microwave-accelerated SPPS procedure described previously on Fmoc-cyclic peptide-Wang resin 72 (316 mg, 100 μmol). Quantities of HBTU, HOBt DIPEA, piperidine and each Fmoc-amino acid were used as described by the automated protocols of the instrument and remained constant throughout this synthesis. The total amount of each coupling reagent and successive amino acid required, along with their reaction duration is summarised in the table below:—

TABLE Quantities of reagents and amino acids used in the synthesis of peptide 73 Total Mass (g) or Reaction Reagent volume (mL) Volume (mL) Time (min) 0.5M HBTU:HOBt in DMF 16 3.03 g:1.08 g — 2M DIPEA in NMP 8    2.8 mL — Fmoc-L-Agl-OH 10 0.675 g 12 Fmoc-L-Ala-OH 5 0.311 g 12 Fmoc-L-Arg(Pbf)-OH 5 0.353 g 12 Fmoc-L-Ile-OH 5 0.353 g 12 Fmoc-L-Pro-OH 5 0.337 g 12 Fmoc-L-Ser(^(t)Bu)—OH 5 0.383 g 12

After sequence completion, the resin-bound peptide was transferred into a fritted syringe and treated with an acetic anhydride solution (7 mL; DMF:acetic anhydride:NMM; 94:5:1) for 2 h. The resin was then washed with DMF (7 mL; 3×1 min), DCM (7 mL; 3×1 min) and MeOH (7 mL; 3×1 min), then left to dry in vacuo for 1 h. Prior to treatment with MeOH, a small aliquot of the resin-bound peptide was removed and subjected to Fmoc-deprotection in the presence of 20% v/v piperidine in DMF (7 mL; 1×1 min, 2×10 min), then washed with DMF (7 mL; 5×1 min), DCM (7 mL; 3×1 min) and MeOH (7 mL; 3×1 min). The aliquot of Fmoc-deprotected resin-tethered peptide was subjected to TFA-mediated cleavage (General Section) for RP-HPLC and mass spectral analysis. This supported formation of the desired sidechain-deprotected peptide 73 in 75% purity. Mass spectrum (ESI⁺, MeCN:H₂O:HCOOH): m/z 496.5 [M+3H]³⁺, ⅓(C₆₆H₁₁₄N₂₁O₁₈) requires 496.3; 744.0 [M+2H]²⁺, ½(C₆₆H₁₁₃N₂₁O₁₈) requires 743.9. RP-HPLC (Vydac C18 analytical column, 0→30% buffer B over 30 min): t_(R)=22.5 min.

c[Δ⁴Das-API]SR-c[Das-API]-SRG 74

Resin-bound Fmoc-protected peptide 73 was subjected to the general microwave-accelerated RCM procedure outlined in the General Section under the following conditions: Resin-bound 73 (393 mg, 100 μmol), DCM (4.75 mL), 0.4 M LiCl in DMF (0.25 mL), 2^(nd) generation Grubbs' catalyst (17 mg, 20 μmol), 100 W pwave, 80° C., 4 h, 100% conversion into 74. Post metathesis, a small aliquot of resin-bound peptide was subjected to Fmoc-deprotection in the presence of 20% v/v piperidine in DMF (7 mL; 1×1 min, 2×10 min), then washed with DMF (7 mL; 5×1 min), DCM (7 mL; 3×1 min) and MeOH (7 mL; 3×1 min). The aliquot of Fmoc-deprotected resin-tethered peptide was subjected to TFA-mediated cleavage (General Section) and RP-HPLC and mass spectral analysis of the resultant isolated solid supported formation of the required unsaturated bicyclic peptide as two isomers, 74(I) and 74(II), in a 3:7 ratio. 74(I): Mass spectrum (ESI⁺, MeCN:H₂O:HCOOH): m/z 487.2 [M+3H]³⁺, ⅓(C₆₄H₁₁₀N₂₁O₁₈) requires 486.9; 730.1 [M+2H]²⁺, ½(C₆₄H₁₀₉N₂₁O₁₈) requires 729.9. RP-HPLC (Vydac C18 analytical column, 0→30% buffer B over 30 min): t_(R)=22.3 min. 74(11): Mass spectrum (ESI⁺, MeCN:H₂O:HCOOH): m/z 787.2 [M+3H]³⁺, ⅓(C₆₄H₁₁₀N₂₁O₁₈) requires 486.9; 730.0 [M+2H]²⁺, ½(C₆₄H₁₀₉N₂₁O₁₈) requires 729.9. RP-HPLC (Vydac C18 analytical column, 0→30% buffer B over 30 min): t_(R)=23.2 min.

c[Das-API]-SR-c[Das-API]-SRG 75

Resin-bound peptide 74 was subjected to the microwave-accelerated hydrogenation procedure described previously under the following conditions: Resin-bound 74 (383 mg, 100 μmol), DCM (4.5 mL), MeOH (0.5 mL), Wilkinson's catalyst, H₂ (80 psi), 100 W pwave, 80° C., 4 h, 100% conversion into 75. Following hydrogenation, a small aliquot of resin-bound peptide was subjected to Fmoc-deprotection in the presence of 20% v/v piperidine in DMF (7 mL; 1×1 min, 2×10 min), then washed with DMF (7 mL; 5×1 min), DCM (7 mL; 3×1 min) and MeOH (7 mL; 3×1 min). The aliquot of Fmoc-deprotected resin-tethered peptide was subjected to TFA-mediated cleavage (General Section) and RP-HPLC and mass spectral analysis of the resultant isolated solid supported formation of the required saturated bicyclic peptide 75. Mass spectrum (ESI⁺, MeCN:H₂O:HCOOH): m/z 487.9 [M+3H]³⁺, ⅓(C₆₄H₁₁₂N₂₁O₁₈) requires 487.6; 731.1 [M+2H]²⁺, ½(C₆₄H₁₁₁N₂₁O₁₈) requires 730.9. RP-HPLC (Vydac C18 analytical column, 0→30% buffer B over 30 min): t_(R)=23.3 min.

AgI-API-AgI-SR-c[Das-API]-SR-c[Das-API]-SRG 76

Synthesis of the extended sequence 76 was performed according to the microwave-accelerated SPPS procedure described previously on Fmoc-bicyclic peptide-Wang resin 75 (376 mg, 100 μmol). Quantities of HBTU, HOBt DIPEA, piperidine and each Fmoc-amino acid were used as described by the automated protocols of the instrument and remained constant throughout this synthesis. The total amount of each coupling reagent and successive amino acid required, along with their reaction duration is summarised in the table below:—

TABLE Quantities of reagents and amino acids used in the synthesis of peptide 76 Total Mass (g) or Reaction Reagent volume (mL) Volume (mL) Time (min) 0.5M HBTU:HOBt in DMF 16 3.03 g:1.08 g — 2M DIPEA in NMP 8    2.8 mL — Fmoc-L-Agl-OH 10 0.675 g 12 Fmoc-L-Ala-OH 5 0.311 g 12 Fmoc-L-Arg(Pbf)-OH 5 0.353 g 12 Fmoc-L-Ile-OH 5 0.353 g 12 Fmoc-L-Pro-OH 5 0.337 g 12 Fmoc-L-Ser(^(t)Bu)—OH 5 0.383 g 12

After sequence completion, the resin-bound peptide was transferred into a fritted syringe and treated with an acetic anhydride solution (7 mL; DMF:acetic anhydride:NMM; 94:5:1) for 2 h. The resin was then washed with DMF (7 mL; 3×1 min), DCM (7 mL; 3×1 min) and MeOH (7 mL; 3×1 min), then left to dry in vacuo for 1 h. Prior to treatment with MeOH, a small aliquot of the resin-bound peptide was removed and subjected to Fmoc-deprotection in the presence of 20% v/v piperidine in DMF (7 mL; 1×1 min, 2×10 min), then washed with DMF (7 mL; 5×1 min), DCM (7 mL; 3×1 min) and MeOH (7 mL; 3×1 min). The aliquot of Fmoc-deprotected resin-tethered peptide was subjected to TFA-mediated cleavage (General Section) for RP-HPLC and mass spectral analysis. This supported formation of the desired sidechain-deprotected extended peptide 76 in 60% purity. Mass spectrum (ESI⁺, MeCN:H₂O:HCOOH): m/z 727.2 [M+3H]³⁺, ⅓(C₉₇H₁₆₆N₃₁O₂₆) requires 727.1; 1090.3 [M+2H]²⁺, ½(C₉₇H₁₆₅N₃₁O₂₆) requires 1090.1. RP-HPLC (Vydac C18 analytical column, 10→30% buffer B over 30 min): t_(R)=20.7 min.

c[Δ⁴Das-API]SR-c[Das-API]-SR-c[Das-API]-SRG 77

Resin-bound Fmoc-protected peptide 76 was subjected to the general microwave-accelerated RCM procedure outlined in the General Section under the following conditions: Resin-bound 76 (422 mg, 100 μmol), DCM (4.75 mL), 0.4 M LiCl in DMF (0.25 mL), 2^(nd) generation Grubbs' catalyst (17 mg, 20 μmol), 100 W μwave, 80° C., 4 h, 100% conversion into 77. Post metathesis, a small aliquot of resin-bound peptide was subjected to Fmoc-deprotection in the presence of 20% v/v piperidine in DMF (7 mL; 1×1 min, 2×10 min), then washed with DMF (7 mL; 5×1 min), DCM (7 mL; 3×1 min) and MeOH (7 mL; 3×1 min). The aliquot of Fmoc-deprotected resin-tethered peptide was subjected to TFA-mediated cleavage (General Section) and RP-HPLC and mass spectral analysis of the resultant isolated solid supported formation of the required unsaturated tricyclic peptide as two isomers, 77(I) and 77(II), in a 1:1 ratio. 77(I): Mass spectrum (ESI⁺, MeCN:H₂O:HCOOH): m/z 539.2 [M+4H]⁴⁺, ¼(C₉₅H₁₆₃N₃₁O₂₆) requires 538.6; 718.3 [M+3H]³⁺, ⅓(C₉₅H₁₆₂N₃₁O₂₆) requires 717.7. RP-HPLC (Vydac C18 analytical column, 10→30% buffer B over 30 min): t_(R)=20.6 min. 77(II): Mass spectrum (ESI⁺, MeCN:H₂O:HCOOH): m/z 538.6 [M+4H]⁴⁺, ¼(C₉₅H₁₆₃N₃₁O₂₆) requires 538.6; 718.1 [M+3H]³⁺, ½(C₉₅H₁₆₂N₃₁O₂₆) requires 717.7. RP-HPLC (Vydac C18 analytical column, 10→30% buffer B over 30 min): t_(R)=21.4 min.

c[Das-API]-SR-c[Das-API]-SR-c[Das-API]-SRG 78

Resin-bound peptide 77 was subjected to the microwave-accelerated hydrogenation procedure described previously under the following conditions: Resin-bound 77 (413 mg, 100 pmol), DCM (4.5 mL), MeOH (0.5 mL), Wilkinson's catalyst, H₂ (80 psi), 100 W pwave, 80° C., 4 h, 100% conversion into 78. Following hydrogenation, a small aliquot of resin-bound peptide was subjected to Fmoc-deprotection in the presence of 20% v/v piperidine in DMF (7 mL; 1×1 min, 2×10 min), then washed with DMF (7 mL; 5×1 min), DCM (7 mL; 3×1 min) and MeOH (7 mL; 3×1 min). The aliquot of Fmoc-deprotected resin-tethered peptide was subjected to TFA-mediated cleavage (General Section) and RP-HPLC and mass spectral analysis of the resultant isolated solid supported formation of the required saturated tricyclic peptide 78. Mass spectrum (ESI⁺, MeCN:H₂O:HCOOH): m/z 539.5 [M+4H]⁴⁺, ¼(C₉₅H₁₆₅N₃₁O₂₆) requires 539.1; 718.7 [M+3H]³⁺, ⅓(C₉₅H₁₆₄N₃₁O₂₆) requires 718.4; 1077.4 [M+2H]²⁺, ½(C₉₅H₁₆₃N₃₁O₂₆) requires 1077.1. RP-HPLC (Vydac C18 analytical column, 10→30% buffer B over 30 min): t_(R)=20.7 min.

Following global Fmoc-deprotection and TFA-mediated cleavage of the remaining peptide 78 from the resin (369 mg), the resultant pale brown solid (128 mg) was purified by RP-HPLC (Agilent: Vydac C18 preparative column, 10→30% buffer B over 30 min, t_(R)=23.3 min). Selected fractions were combined and lyophilised to give the desired peptide 78 as a colourless solid (3.8 mg, 2%) in 90% purity. Spectral data were consistent with those reported previously.

Preparation of Nva-API-Nva-SLG 85 AgI-SLG 80

The automated, microwave-accelerated procedure outlined in the General Section was used for the synthesis of peptide 80 on Fmoc-Gly-Wang resin (0.94 mg, 0.50 mmol). Quantities of HATU, DIPEA, piperidine and each Fmoc-amino acid were used as described by the automated protocols of the instrument and remained constant throughout this synthesis. The total amount of each coupling reagent and successive amino acid required, along with their reaction duration is summarised in the table below:—

TABLE Quantities of reagents and amino acids used in the synthesis of peptide 80 Total Mass (g) or Reaction Reagent volume (mL) Volume (mL) Time (min) 0.5M HATU in DMF 14 2.67 g — 2M DIPEA in NMP 7   2.4 mL — Fmoc-L-Agl-OH 11 0.74 g 5 Fmoc-L-Leu-OH 11 0.78 g 5 Fmoc-L-Ser(^(t)Bu)—OH 11 0.84 g 5

After sequence completion, the resin-bound peptide was transferred into a fritted syringe and treated with an acetic anhydride solution (7 mL; DMF:acetic anhydride:NMM; 94:5:1) for 2 h. The resin was then washed with DMF (7 mL; 3×1 min), DCM (7 mL; 3×1 min) and MeOH (7 mL; 3×1 min), then left to dry in vacuo for 1 h. A small aliquot of the resin-tethered peptide was subjected to TFA-mediated cleavage (General Section) for RP-HPLC and mass spectral analysis which supported formation of the desired peptide 80 in 97% purity. Mass spectrum (ESI⁺, MeCN:H₂O:HCOOH): m/z 595.3 [M+H]⁺, C₃₁H₃₉N₄O₈ requires 595.3. RP-HPLC (Vydac C18 analytical column, 0→100% buffer B over 30 min): t_(R)=17.0 min.

Crt-SLG 81

Resin-bound peptide 80 was subjected to the conventional CM procedure outlined in the General Section under the following conditions: Resin-bound 80 (0.71 mg, 0.33 mmol), DCM (8 mL), 2^(nd) generation Grubbs' catalyst (57 mg, 67 μmol), cis-2-butene (12 psi), A, 20 h, 97% conversion into 81. Post metathesis a small aliquot of resin-bound peptide was subjected to TFA-mediated cleavage (General Section), and RP-HPLC and mass spectral analysis of the resultant solid supported formation of the desired peptide 81. A homodimer by-product resulting from CM of the resin-bound sequence was also detected in the crude mass spectrum. Mass spectrum (ESI⁺, MeCN:H₂O:HCOOH): m/z 595.1; 609.2 [M+H]⁺, C₃₂H₄₁N₄O₈ requires 609.3; 1161.2. RP-HPLC (Vydac C18 analytical column, 0→100% buffer B over 30 min): t_(R)=17.7 min.

Nva-SLG 82

Resin-bound peptide 82 was subjected to the conventional hydrogenation procedure outlined in the General Section under the following conditions: Resin-bound 81 (0.72 mg, 0.33 mmol), DCM (9 mL), MeOH (1 mL), Wilkinson's catalyst, H₂ (90 psi), r.t., 16 h, 100% conversion into 82. Following hydrogenation a small aliquot of the resin-bound peptide was subjected to TFA-mediated cleavage (General Section), and RP-HPLC and mass spectral analysis of the resultant solid supported formation of the desired saturated sidechain in peptide 82. Mass spectrum (ESI⁺, MeCN:H₂O:HCOOH): m/z 611.3 [M+H]⁺, C₃₂H₄₃N₄O₈ requires 611.3. RP-HPLC (Vydac C18 analytical column, 0→100% buffer B over 30 min): t_(R)=18.1 min.

AgI-API-Nva-SLG 83

Synthesis of the extended sequence 83 was performed according to the microwave-accelerated SPPS procedure described in the General Section on Fmoc-peptide-Wang resin 82 (0.71 g, 0.33 mmol). Quantities of HATU, DIPEA, piperidine and each Fmoc-amino acid were used as described by the automated protocols of the instrument and remained constant throughout this synthesis. The total amount of each coupling reagent and successive amino acid required, along with their reaction duration is summarised in the table below:—

TABLE Quantities of reagents and amino acids used in the synthesis of peptide 83 Total Mass (g) or Reaction Reagent volume (mL) Volume (mL) Time (min) 0.5M HATU in DMF 19 3.62 g — 2M DIPEA in NMP 9   3.1 mL — Fmoc-L-Agl-OH 11 0.74 5 Fmoc-L-Ala-OH 11 0.68 5 Fmoc-L-Ile-OH 11 0.78 5 Fmoc-L-Pro-OH 11 0.74 5

After sequence completion, the resin-bound peptide was transferred into a fritted syringe and treated with an acetic anhydride solution (4 mL; DMF:acetic anhydride:NMM; 94:5:1) for 2 h. The resin was then washed with DMF (4 mL; 3×1 min), DCM (4 mL; 3×1 min) and MeOH (4 mL; 3×1 min), then left to dry in vacuo for 1 h. Prior to treatment with MeOH, a small aliquot of the resin-bound peptide was removed and subjected to Fmoc-deprotection in the presence of 20% v/v piperidine in DMF (1 mL; 1×1 min, 2×10 min), then washed with DMF (1 mL; 5×1 min), DCM (1 mL; 3×1 min) and MeOH (1 mL; 3×1 min). The dried aliquot of Fmoc-deprotected resin-tethered peptide was subjected to TFA-mediated cleavage (General Section) for RP-HPLC and mass spectral analysis. This supported formation of the desired peptide 83 in 94% purity. Mass spectrum (ESI⁺, MeCN:H₂O:HCOOH): m/z 767.5 [M+H]⁺, C₃₆H₆₃N₈O₁₀) requires 767.5. RP-HPLC (Vydac C18 analytical column, 15→45% buffer B over 30 min): t_(R)=12.8 min.

Crt-API-Nva-SLG 84

Resin-bound peptide 83 was subjected to the conventional CM procedure outlined in the General Section under the following conditions: Resin-bound 83 (385 mg, 167 μmol), DCM (5 mL), 2^(nd) generation Grubbs' catalyst (28 mg, 33 μmol), cis-2-butene (12 psi), A, 16 h, 96% conversion into 84. Post metathesis a small aliquot of the resin-bound peptide was removed and subjected to Fmoc-deprotection in the presence of 20% v/v piperidine in DMF (1 mL; 1×1 min, 2×10 min), then washed with DMF (1 mL; 5×1 min), DCM (1 mL; 3×1 min) and MeOH (1 mL; 3×1 min). The dried aliquot of Fmoc-deprotected resin-tethered peptide was subjected to TFA-mediated cleavage (General Section) for RP-HPLC and mass spectral analysis. This supported formation of the desired crotylglycine-containing peptide 84. Mass spectrum (ESI⁺, MeCN:H₂O:HCOOH): m/z 767.6; 781.6 [M+H]⁺, C₃₇H₆₅N₈O₁₀ requires 781.5. RP-HPLC (Vydac C18 analytical column, 15→45% buffer B over 30 min): t_(R)=14.2 min.

Nva-API-Nva-SLG 85

Resin-bound peptide 84 was subjected to the microwave-accelerated hydrogenation procedure described in the General Section under the following conditions: Resin-bound 84 (389 mg, 167 μmol), DCM (4.5 mL), MeOH (0.5 mL), Wilkinson's catalyst, H₂ (90 psi), 100 W pwave, 100° C., 1 h, 100% conversion into 85. Following hydrogenation, a small aliquot of resin-bound peptide was subjected to Fmoc-deprotection in the presence of 20% v/v piperidine in DMF (1 mL; 1×1 min, 2×10 min), then washed with DMF (1 mL; 5×1 min), DCM (1 mL; 3×1 min) and MeOH (1 mL; 3×1 min). The dried aliquot of Fmoc-deprotected resin-tethered peptide was subjected to TFA-mediated cleavage (General Section) and RP-HPLC and mass spectral analysis of the resultant isolated solid supported formation of the required bis-norvaline-containing peptide 85. Mass spectrum (ESI⁺, MeCN:H₂O:HCOOH): m/z 769.6; 783.7 [M+H]⁺, C₃₇H₆₇N₈O₁₀ requires 783.5. RP-HPLC (Vydac C18 analytical column, 15→45% buffer B over 30 min): t_(R)=15.2 min.

Preparation of c[Das-SL]-API-Nva-SLG 92 AgI-SL-AgI-API-Nva-SLG 90

Synthesis of the extended sequence 90 was performed according to the microwave-accelerated SPPS procedure described in the General Section on Fmoc-peptide-Wang resin 83 (390 g, 167 μmol). Quantities of HATU DIPEA, piperidine and each Fmoc-amino acid were used as described by the automated protocols of the instrument and remained constant throughout this synthesis. The total amount of each coupling reagent and successive amino acid required, along with their reaction duration is summarised in the table below:—

TABLE Quantities of reagents and amino acids used in the synthesis of peptide 90 Total Mass (g) or Reaction Reagent volume (mL) Volume (mL) Time (min) 0.5M HATU in DMF 7 1.33 g — 2M DIPEA in NMP 4   1.4 mL — Fmoc-L-Agl-OH 6 0.40 g 12 Fmoc-L-Leu-OH 6 0.42 g 12 Fmoc-L-Ser(^(t)Bu)—OH 6 0.46 g 12

After sequence completion, the resin-bound peptide was transferred into a fritted syringe and treated with an acetic anhydride solution (4 mL; DMF:acetic anhydride:NMM; 94:5:1) for 2 h. The resin was then washed with DMF (4 mL; 3×1 min), DCM (4 mL; 3×1 min) and MeOH (4 mL; 3×1 min), then left to dry in vacuo for 1 h. Prior to treatment with MeOH, a small aliquot of the resin-bound peptide was removed and subjected to Fmoc-deprotection in the presence of 20% v/v piperidine in DMF (1 mL; 1×1 min, 2×10 min), then washed with DMF (1 mL; 5×1 min), DCM (1 mL; 3×1 min) and MeOH (1 mL; 3×1 min). The dried aliquot of Fmoc-deprotected resin-tethered peptide was subjected to TFA-mediated cleavage (General Section) for RP-HPLC and mass spectral analysis. This supported formation of the desired peptide 90 in 90% purity. Mass spectrum (ESI⁺, MeCN:H₂O:HCOOH): m/z 532.9 [M+2H]²⁺, C₅₀H₈₇N₁₁O₁₄ requires 532.8; 1064.8 [M+H]⁺, C₅₀H₈₆N₁₁O₁₄ requires 1064.6. RP-HPLC (Vydac C18 analytical column, 15→45% buffer B over 30 min): t_(R)=12.8 min.

c[Δ⁴Das-SL]-API-Nva-SLG 91

Resin-bound Fmoc-protected peptide 90 was subjected to the microwave-accelerated RCM procedure outlined in the General Section under the following conditions: Resin-bound 90 (403 mg, 167 μmol), DCM (4.75 mL), 0.4 M LiCl in DMF (0.25 mL), 2^(nd) generation Grubbs' catalyst (28 mg, 33 μmol), 100 W pwave, 100° C., 2 h, 100% conversion into 91. Post metathesis, a small aliquot of resin-bound peptide was subjected to Fmoc-deprotection in the presence of 20% v/v piperidine in DMF (1 mL; 1×1 min, 2×10 min), then washed with DMF (1 mL; 5×1 min), DCM (1 mL; 3×1 min) and MeOH (1 mL; 3×1 min). The dried aliquot of Fmoc-deprotected resin-tethered peptide was subjected to TFA-mediated cleavage (General Section), and RP-HPLC and mass spectral analysis of the resultant isolated solid supported formation of the required peptide as two isomers, 91(I) and 91(II), in a 55:45 ratio. 91(I): Mass spectrum (ESI⁺, MeCN:H₂O:HCOOH): m/z 518.9 [M+2H]²⁺, C₄₈H₈₃N₁₁O₁₄ requires 518.8; 1036.7 [M+H]⁺, C₄₈H₈₂N₁₁O₁₄ requires 1036.6. RP-HPLC (Vydac C18 analytical column, 15→45% buffer B over 30 min): t_(R)=16.4 min. 91(II): Mass spectrum (ESI⁺, MeCN:H₂O:HCOOH): m/z 518.9 [M+2H]²⁺, C₄₈H₈₃N₁₁O₁₄ requires 518.8; 1036.7 [M+H]⁺, C₄₈H₈₂N₁₁O₁₄ requires 1036.6. RP-HPLC (Vydac C18 analytical column, 15→45% buffer B over 30 min): t_(R)=16.6 min.

c[Das-SL]-API-Nva-SLG 92

Resin-bound peptide 91 was subjected to the microwave-accelerated hydrogenation procedure described in the General Section under the following conditions: Resin-bound 91 (395 mg, 167 μmol), DCM (4.5 mL), MeOH (0.5 mL), Wilkinson's catalyst, H₂ (80 psi), 80 W pwave, 80° C., 4 h, 100% conversion into 92. Following hydrogenation, a small aliquot of resin-bound peptide was subjected to Fmoc-deprotection in the presence of 20% v/v piperidine in DMF (1 mL; 1×1 min, 2×10 min), then washed with DMF (1 mL; 5×1 min), DCM (1 mL; 3×1 min) and MeOH (1 mL; 3×1 min). The dried aliquot of Fmoc-deprotected resin-tethered peptide was subjected to TFA-mediated cleavage (General Section), and RP-HPLC and mass spectral analysis of the resultant isolated solid supported formation of the required saturated carbocycle 92. Mass spectrum (ESL MeCN:H₂O:HCOOH): m/z 519.8 [M+2H]²⁺, C₄₈H₈₅N₁₁O₁₄ requires 519.8; 1038.7 [M+H]⁺, C₄₈H₈₄N₁₁O₁₄ requires 1038.6. RP-HPLC (Vydac C18 analytical column, 15→45% buffer B over 30 min): t_(R)=16.5 min.

Preparation of Ugl-API-Das (±N-Ac-Das-OMe)-SLG 103 Δ⁴Das((±)N-Ac-Δ⁴Das-OMe)-SLG 100

Resin-bound peptide 80 was subjected to the microwave-accelerated CM procedure outlined in the General Section under the following conditions: Resin-bound 80 (220 mg, 100 μmol), DCM (4 mL), 2^(nd) generation Hoveyda-Grubbs' catalyst (13 mg, 20 μmol), (±)-N-Ac-Agl-OMe (0.17 g, 1.0 mmol), 80 W pwave, 80° C., 4 h, 58% conversion into 100. Post metathesis a small aliquot of resin-bound peptide was subjected to TFA-mediated cleavage (General Section), and RP-HPLC and mass spectral analysis of the resultant solid supported formation of the desired peptide 100. Linear peptide 80 and homodimer, resulting from CM of this resin-bound sequence, was also detected in the crude spectrum. Mass spectrum (ESI⁺, MeCN:H₂O:HCOOH): m/z 595.2; 738.3 [M+H]⁺, C₃₇H₄₈N₅O₁₁ requires 738.3; 1161.3. RP-HPLC (Vydac C18 analytical column, 0→100% buffer B over 30 min): t_(R)=15.8 min.

Das((±)N-Ac-Das-OMe)-SLG 101

Resin-bound peptide 100 was subjected to the conventional hydrogenation procedure outlined in the General Section under the following conditions: Resin-bound 100 (222 mg, 100 μmol), DCM (4.5 mL), MeOH (0.5 mL), Wilkinson's catalyst, H₂ (90 psi), r.t., 16 h, 100% conversion into 101. Following hydrogenation a small aliquot of the resin-bound peptide was subjected to TFA-mediated cleavage (General Section), and RP-HPLC and mass spectral analysis of the resultant solid supported formation of the desired saturated sidechain in peptide 101. Mass spectrum (ESI⁺, MeCN:H₂O:HCOOH): m/z 597.3; 740.3 [M+H]⁺, C₃₇H₅₀N₅O₁₁ requires 740.4; 1163.4. RP-HPLC (Vydac C18 analytical column, 0→100% buffer B over 30 min): t_(R)=15.8 min.

AgI-API-Das((±)N-Ac-Das-OMe)-SLG 102

Synthesis of the extended sequence 102 was performed according to the microwave-accelerated SPPS procedure described in the General Section on Fmoc-peptide-Wang resin 101 (218 mg, 100 μmol). Quantities of HATU, DIPEA, piperidine and each Fmoc-amino acid were used as described by the automated protocols of the instrument and remained constant throughout this synthesis. The total amount of each coupling reagent and successive amino acid required, along with their reaction duration is summarised in the table below:—

TABLE Quantities of reagents and amino acids used in the synthesis of peptide 102 Total Mass (g) or Reaction Reagent volume (mL) Volume (mL) Time (min) 0.5M HATU in DMF 5 0.95 g — 2M DIPEA in NMP 3   1.0 mL — Fmoc-L-Agl-OH 5 0.34 g 12 Fmoc-L-Ala-OH 5 0.31 g 12 Fmoc-L-Ile-OH 5 0.35 g 12 Fmoc-L-Pro-OH 5 0.34 g 12

After sequence completion, the resin-bound peptide was transferred into a fritted syringe and treated with an acetic anhydride solution (4 mL; DMF:acetic anhydride:NMM; 94:5:1) for 2 h. The resin was then washed with DMF (4 mL; 3×1 min), DCM (4 mL; 3×1 min) and MeOH (4 mL; 3×1 min), then left to dry in vacuo for 1 h. Prior to treatment with MeOH, a small aliquot of the resin-bound peptide was removed and subjected to Fmoc-deprotection in the presence of 20% v/v piperidine in DMF (1 mL; 1×1 min, 2×10 min), then washed with DMF (1 mL; 5×1 min), DCM (1 mL; 3×1 min) and MeOH (1 mL; 3×1 min). The dried aliquot of Fmoc-deprotected resin-tethered peptide was subjected to TFA-mediated cleavage (General Section) for RP-HPLC and mass spectral analysis. This supported formation of the desired peptide 102 in 57% purity. Mass spectrum (ESI⁺, MeCN:H₂O:HCOOH): m/z 753.6; 896.6 [M+H]⁺, C₄₁H₇₀N₉O₁₃ requires 896.5. RP-HPLC (Vydac C18 analytical column, 15→45% buffer B over 30 min): t_(R)=8.5 and 8.7 min.

Δ⁴ Ugl-API-Das((±)N-Ac-Das-OMe)-SLG 103

Resin-bound peptide 102 was subjected to the microwave-accelerated CM procedure outlined in the General Section under the following conditions: Resin-bound 102 (130 mg, 50 μmol), DCM (3 mL), 2^(nd) generation Hoveyda-Grubbs' catalyst (6.3 mg, 10 μmol), decene (95 μL, 0.5 mmol), 100 W pwave, 1 h, 95% conversion into 103. Post metathesis a small aliquot of the resin-bound peptide was removed and subjected to Fmoc-deprotection in the presence of 20% v/v piperidine in DMF (1 mL; 1×1 min, 2×10 min), then washed with DMF (1 mL; 5×1 min), DCM (1 mL; 3×1 min) and MeOH (1 mL; 3×1 min). The dried aliquot of Fmoc-deprotected resin-tethered peptide was subjected to TFA-mediated cleavage (General Section) for RP-HPLC and mass spectral analysis. This supported formation of the desired long chain alkene-containing peptide 103 in addition to isomerised by-products. Mass spectrum (ESI⁺, MeCN:H₂O:HCOOH): m/z 767.5; 771.4; 896.6; 910.6; 924.6; 938.7; 952.7; 966.7; 980.7; 1008.7 [M+H]⁺, C₄₉H₈₆N₉O₁₃ requires 1008.6.

Preparation of c[Das-SL]-API-Das((±)N-Ac-Das-OMe)-SLG 112 AgI-SL-AgI-API-Das((±)N-Ac-Das-OMe)-SLG 110

Synthesis of the extended sequence 110 was performed according to the microwave-accelerated SPPS procedure described in the General Section on Fmoc-peptide-Wang resin 102 (119 mg, 50 μmol). Quantities of HATU DIPEA, piperidine and each Fmoc-amino acid were used as described by the automated protocols of the instrument and remained constant throughout this synthesis. The total amount of each coupling reagent and successive amino acid required, along with their reaction duration is summarised in the table below:—

TABLE Quantities of reagents and amino acids used in the synthesis of peptide 110 Total Mass (g) or Reaction Reagent volume (mL) Volume (mL) Time (min) 0.5M HATU in DMF 4 0.76 g — 2M DIPEA in NMP 2   0.7 mL — Fmoc-L-Agl-OH 5 0.34 g 12 Fmoc-L-Leu-OH 5 0.35 g 12 Fmoc-L-Ser(^(t)Bu)—OH 5 0.38 g 12

After sequence completion, the resin-bound peptide was transferred into a fritted syringe and treated with an acetic anhydride solution (4 mL; DMF:acetic anhydride:NMM; 94:5:1) for 2 h. The resin was then washed with DMF (4 mL; 3×1 min), DCM (4 mL; 3×1 min) and MeOH (4 mL; 3×1 min), then left to dry in vacuo for 1 h. Prior to treatment with MeOH, a small aliquot of the resin-bound peptide was removed and subjected to Fmoc-deprotection in the presence of 20% v/v piperidine in DMF (1 mL; 1×1 min, 2×10 min), then washed with DMF (1 mL; 5×1 min), DCM (1 mL; 3×1 min) and MeOH (1 mL; 3×1 min). The dried aliquot of Fmoc-deprotected resin-tethered peptide was subjected to TFA-mediated cleavage (General Section) for RP-HPLC and mass spectral analysis. This supported formation of the desired peptide 110 in 60% purity. Mass spectrum (ESI⁺, MeCN:H₂O:HCOOH): m/z 597.5 [M+2H]²⁺, C₅₅H₉₃N₁₂O₁₇ requires 597.3; 1050.8; 1194.0 [M+H]⁺, C₅₅H₉₃N₁₂O₁₇ requires 1193.7. RP-HPLC (Vydac C18 analytical column, 15→45% buffer B over 30 min): t_(R)=15.7 and 16.0 min.

c[Δ⁴Das-SL]-API-Das((±)N-Ac-Das-OMe)-SLG 111

Resin-bound Fmoc-protected peptide 110 was subjected to the microwave-accelerated RCM procedure outlined in the General Section under the following conditions: Resin-bound 110 (126 mg, 50 μmol), DCM (4.75 mL), 0.4 M LiCl in DMF (0.25 mL), 2^(nd) generation Grubbs' catalyst (8.5 mg, 10 μmol), 100 W pwave, 100° C., 2 h, 98% conversion into 111. Post metathesis, a small aliquot of resin-bound peptide was subjected to Fmoc-deprotection in the presence of 20% v/v piperidine in DMF (1 mL; 1×1 min, 2×10 min), then washed with DMF (1 mL; 5×1 min), DCM (1 mL; 3×1 min) and MeOH (1 mL; 3×1 min). The dried aliquot of Fmoc-deprotected resin-tethered peptide was subjected to TFA-mediated cleavage (General Section), and RP-HPLC and mass spectral analysis of the resultant isolated solid supported formation of the required peptide 111. Mass spectrum (ESI⁺, MeCN:H₂O:HCOOH): m/z 583.5 [M+2H]²⁺, C₅₃H₉₀N₁₂O₁₇ requires 583.3; 1022.7; 1165.9 [M+H]⁺, C₅₃H₈₉N₁₂O₁₇ requires 1165.6. RP-HPLC (Vydac C18 analytical column, 15→45% buffer B over 30 min): t_(R)=11.7 and 12.0 min.

c[Das-SL]-API-Das((±)N-Ac-Das-OMe)-SLG 112

Resin-bound peptide 111 was subjected to the microwave-accelerated hydrogenation procedure described in the General Section under the following conditions: Resin-bound 111 (109 mg, 50 μmol), DCM (4.5 mL), MeOH (0.5 mL), Wilkinson's catalyst, H₂ (90 psi), 80 W pwave, 80° C., 4 h, 100% conversion into 112. Following hydrogenation, a small aliquot of resin-bound peptide was subjected to Fmoc-deprotection in the presence of 20% v/v piperidine in DMF (1 mL; 1×1 min, 2×10 min), then washed with DMF (1 mL; 5×1 min), DCM (1 mL; 3×1 min) and MeOH (1 mL; 3×1 min). The dried aliquot of Fmoc-deprotected resin-tethered peptide was subjected to TFA-mediated cleavage (General Section), and RP-HPLC and mass spectral analysis of the resultant isolated solid supported formation of the required saturated carbocycle 112. Mass spectrum (ESI⁺, MeCN:H₂O:HCOOH): m/z 584.5 [M+2H]²⁺, C₅₃H₉₂N₁₂O₁₇ requires 584.3; 1024.7; 1167.9 [M+H]⁺, C₅₃H₉₁N₁₂O₁₇ requires 1167.7. RP-HPLC (Vydac C18 analytical column, 15→45% buffer B over 30 min): t_(R)=12.5 and 12.6 min.

In this specification, including the claims which follow, except where the context requires otherwise due to express language or necessary implication, the word “comprising” or variations such as “comprise” or “comprises” is used in the inclusive sense, to specify the presence of the stated features or steps but not to preclude the presence or addition of further features or steps.

As used in the specification, the words “a”, “an” and “the” include the plural equivalents, unless the context clearly indicates otherwise. Thus, for example, reference to “an amino acid” or “a dicarba bridge” includes one or more amino acids, or one or more dicarba bridges, respectively. 

1. A method for preparing a peptide or peptides containing a dicarba bridge, comprising: (i) providing a reactable peptide having at least two complementary metathesisable groups or two or more reactable peptides having at least two complementary metathesisable groups between them; (ii) subjecting the reactable peptide or reactable peptides to metathesis to form a reactable peptide or peptides having at least one unsaturated dicarba bridge; (iii) reducing at least one unsaturated dicarba bridge of the reactable peptide or peptides to form a saturated dicarba bridge; and (iv) adding one or more further amino acids to one or both ends of at least one of the reactable pe flpc le or peptides.
 2. The method of claim 1, wherein in step (iv) the one or more further amino acids are added to the N-terminus of at least one of the peptide or peptides.
 3. The method of claim 1, wherein in step (iv) the one or more further amino acids are added by peptide synthesis.
 4. The method of claim 1, wherein at least one reactable peptide is provided on a solid support.
 5. (canceled)
 6. (canceled)
 7. The method of claim 1, wherein the reducing involves either hydrogenation of at least one dicarba bridge or hydrosilylation and protodesilylation of at least one dicarba bridge.
 8. (canceled)
 9. The method of claim 1, wherein step (iv) comprises adding two or more further amino acids to one or both ends of at least one of the reactable peptide or peptides, wherein at least two of the further amino acids have complementary metathesisable groups.
 10. The method of claim 9, further comprising subjecting the at least one reactable peptide or peptides to a further step of metathesis to form a peptide or peptides having at least two dicarba bridges.
 11. A method for preparing a peptide containing a plurality of dicarba bridges, comprising: i) providing a reactable peptide having at least two complementary metathesisable groups; (ii) subjecting the reactable peptide to metathesis to form a peptide having at least one unsaturated dicarba bridge; (iii) adding one or more further amino acids comprising at least two complementary metathesisable groups to one or both ends of the peptide; and (iv) repeating steps (ii) and (iii) at least once to prepare the peptide containing a plurality of unsaturated dicarba bridges.
 12. The method of claim 11, wherein at least a part of the peptide containing a plurality of unsaturated dicarba bridges is α-helical in structure.
 13. The method of claim 11, wherein the reactable peptide comprises a turn inducing residue located between each of the two complementary metathesisable groups.
 14. The method of claim 1, wherein the reactable peptide comprises a removable tether between the two complementary metathesisable groups, and the method further comprises the step of removing the removeable tether to provide a peptide containing an intermolecular dicarba bridge.
 15. The method of claim 1, wherein the reactable peptide has three or more complementary metathesisable groups, or at least two reactable peptides have three or more complementary metathesisable groups between them, where two of the complementary metathesisable groups are unblocked and form the dicarba bridge in step (ii), while the other metathesisable group or groups are blocked.
 16. The method of claim 15, further comprising the step of unblocking at least one blocked metathesisable group.
 17. The method of claim 16, further comprising the step of adding at least one further amino acid having an unblocked metathesisable group to one or both ends of the reactable peptide or peptides, and subjecting the unblocked metathesisable groups to a further metathesis step to form a further dicarba bridge.
 18. The method of claim 16, further comprising the step of adding at least one further reactable peptide having at least one unblocked metathesisable group, and subjecting the reactable peptides to metathesis to form a further dicarba bridge.
 19. The dicarba analogue according to claim 1, wherein the unsaturated dicarba bridge from step (ii) is selected from the group consisting of:

wherein R₁ to R₆ are each independently absent or selected from a divalent linking group.
 20. The dicarba analogue according to claim 1, wherein the saturated dicarba bridge from step (iii) is selected from the group consisting of:

wherein R₁ to R₆ are each independently absent or selected from a divalent linking group.
 21. The method of claim 1, wherein the complementary metathesisable groups are alkyne-containing metathesisable groups of the formula:

in which n is an integer between 0 and 10, R₇ is substituted or unsubstituted alkyl and R₈ is hydrogen, substituted or unsubstituted alkyl or a further alkyne-containing metathesisable group or an alkene-containing metathesisable group.
 22. The method of claim 1, wherein the complementary metathesisable groups are alkene-containing metathesisable groups of the formula:

in which n is an integer between 0 and 10, R₉ and R₁₀ are each independently either H or substituted or unsubstituted alkyl and R₁₁ is hydrogen, substituted or unsubstituted alkyl or a further alkene-containing metathesisable group or an alkyne-containing metathesisable group.
 23. The method of claim 1, wherein the complementary metathesisable groups are located on an amino group or a side chain of an amino acid.
 24. The method of claim 23, wherein the amino acid having the complementary metathesisable groups are selected from the group consisting of butynylglycine, allylglycine, prenylglycine and crotylglycine.
 25. A peptide or peptides with at least one dicarba bridge when synthesised by the method of claim
 1. 26. A peptide or peptides with at least one dicarba bridge when synthesised by the method of claim
 11. 